Guess who's coming to dinner? Meet your surprise holiday guest: Physics! (Physical/tech science: light waves/forces & motion/chemistry).

The last thing on your mind when you sink your teeth into a perfectly roasted drumstick may he science. "But when you start asking questions about the world, you'll find science everywhere," says physicist Lou Bloomfield. fine of the best places to look is right on your dinner plate.

(1) Inspector Gadget

This is your job: As dangling poultry carcasses glide past on a production line, you turkey trot alongside them, inspecting 35 birds a minute. You have two seconds to spy any visible defect or disease, which would make the bird unfit to eat. After an eight-hour shift, your glazed eyes have pored over 16,800 dead chickens! Having fun yet?

Engineer Yud Ren Chen doesn't think so. That's why his team at the Agricultural Research Service in Beltsville, Md., whipped up an invention to lighten a poultry inspector's load. The robotic inspection system is called "machine vision," and "sees" with two digital cameras, a light probe, and a spectrophotometer (device that measures light intensity at different wavelengths). The gadget scans 140 birds a minute and instantly feeds data into a computer.

One camera shoots the bird's front; the other, its back. Like human eyes, each camera detects light waves, rays of energy that travel through space and matter (see diagram, above). But while human eyes can miss subtle color abnormalities on a bird's skin, the cameras catch it all: "We designed special light filters for the lenses to reveal features like bruises, tumors, or bloody flesh," Chen says.

Robots may command superior vision, but humans ultimately determine which birds are edible. When the computer spies a defective carcass, a red light flashes and a "live" inspector reexamines the bird.

Machine vision is slated for testing in poultry plants this month.

(2) Sop Science

Leftover gravy isn't the world's most pressing problem, but it is common: 150,000 gallons per week are trashed in England alone, says Len Fisher, author of the book How to Dunk a Doughnut. He deployed the scientific method to find out which foods absorb the most gravy. By plunging meat, veggies, and breads into gravy, then removing and weighing each item at time intervals, Fisher hit on a gravy equation:

PERCENT GRAVY UPTAKE = (W-(D/S))/D x 100

W = uncooked weight of the food

D = cooked weight of the food difference

S = shrinkage, or the weight d' between cooked and uncooked food

Using his equation, Fisher found that ciabatta, an airy Italian bread, boasted a whopping 120 percent gravy uptake, almost double the absorption of plain white bread. What makes ciabatta a super sponge? The unique way microscopic particles hold the bread together. Basically, "there's a strong chemical attraction between the bread and gravy," explains University of Virginia physicist Lou Bloomfield.

Consider gravy. It's up to 90 percent water, a molecule with two atoms of hydrogen and one atom of oxygen. The ends of the molecule are polar: like a battery, the hydrogen end carries a positive charge; the oxygen end, negative. Since opposite charges attract, water's negative poles grab the positive poles of other molecules, like hydrogen in carbohydrates such as breads. Carbohydrates are nutrients made of long chains of carbon, oxygen, and hydrogen molecules, explains Bloomfield.

And ciabatta, unlike your basic white bread, is riddled with cavernous air holes, which help funnel more gravy into the bread and leave less on your plate!

(3) Ketchup Trouble

Do you like ketchup on everything--including the holiday roast? Then you know the trouble with this tasty condiment. One second it won't flow, the next it gushes like lava.

Welcome to the unusual world of pseudoplastic liquids, like ketchup, whipped cream, and toothpaste. The viscosity--or thickness--of these liquids depends on the amount of stress they're subjected to. "Ketchup behaves like a solid until enough force acts on it," says Massachusetts Institute of Technology mechanical engineer Gareth McKinley. "Then it flows as a liquid."

How much force--pushing or pulling action--does it take to move ketchup? The answer depends on its yield stress, the minimum amount of stress needed to change the liquid's shape. Give the bottle a whack greater than ketchup's yield stress, and get out of the way! "Nothing is happening, then all of a sudden it moves too fast," explains NASA physicist Greg Zimmerli.

The reason for such behavior? Ketchup is made of many ingredients stirred together. In a bottle, the ground tomatoes form a gel that thickens over time. The best way to crack the gel: "A few gentle whacks on the back or neck of the bottle and a little patience," says Zimmerli. Or, suggests McKinley, try a squeeze bottle!

(4) Nuts for Physics

A bowl of mixed nuts may be prime holiday snack food, but for physicist Paul Quinn it's a nutty physics project. Quinn was puzzled by an odd nut-bowl phenomenon: Chunky Brazil nuts always seem to sit on the top of smaller peanuts, almonds, and cashews. But shouldn't gravity's downward force pull the heavy nuts--those with more matter, or mass--to the bowl's bottom, while lighter nuts rest on top? Quinn calls the phenomenon, the Brazil-nut problem, or BNP. And the shuffling can occur in any granular mixture--from trail mix to cement.

How to explain BNP? "In some cases little nuts Simply fall through the cracks and weasel their way under big ones," says Quinn, a professor at Kutztown University in Pennsylvania. In other cases, a convection-like effect drives the chubby nuts to the top of the mix. Convection describes how heat flows through liquid or gas. When water boils, for example, heat causes the water molecules to expand, become less dense, and rise up to mix with cooler water molecules. The same principle (minus the heat) works on mixed nuts, says Quinn.

His latest research offers another look at how convection's lift and gravity's pull affect mixed nuts. Quinn found that a nut sinks or swims depending on the ratio of two properties: mass and diameter (distance across a circle's center). If a fat nut is twice the mass and diameter of the other nuts in the bowl, it surfaces. But if the nut is six times the mass and only twice the diameter of smaller nuts, it sinks. "I call this `the reverse BNP,'" Quinn says.

WHAT'S PHYSICS DOING AT THE DINNER TABLE?

Take this True/False quiz. Read the article for' the answers.

1. Robotic machines will soon use gamma rays to scan turkeys for defects.

(True) (False)

2. There's a scientific theory behind mopping up gravy with bread.

(True) (False)

3. The most efficient way to get ketchup out of the bottle is to hold it straight upside down and wait.

(True) (False)

4. When you shake a bowl of nuts, the biggest ones wind up on tap because of gravity's pull.

(True) (False)

FOR ANSWERS, SEE P. 16

PROBING POULTRY: USING THE ELECTROMAGNETIC SPECTRUM 

Light waves are one of seven major types of energy waves, categorized on the electromagnetic spectrum according to decreasing wavelength, or distance between two waves, Visible light--the only energy waves you can see--is composed of colors from red to violet. Engineer Yud Ren Chen's "machine vision" system deters visible and near-infrared light by reading their signature wavelengths--red having the longest wave length and violet the shortest.

[ILLUSTRATION OMITTED]

HANDS-ON SCIENCE

NUTQUAKE!

* You Need:

1-quart glass jar with lid * stopwatch * measuring spoon * measuring cup * 1 can mixed nuts * 1 box Raisin Bran cereal * 3 cups salt * 5 large marbles * 1 teaspoon long-grain white rice * paper * pencil

* To Do: Between each of the following four separate trials, clean and dry the jar.

1 Pour the mixed nuts into the jar and screw on lid. As you shake the jar, have a partner time how long it takes for the Brazil nuts to rise to the top. Record time in a data table.

2 Pour Raisin Bran into jar and screw on lid. As you shake the jar, have a partner time how long it takes for the raisins to rise to the top. Record time in a data table.

3 Put marbles in jar. Pour in I cup salt. Cover jar and shake until marbles rise to top. Record time.

4 Put I teaspoon of rice in the bottom of the jar. Pour 1 cup salt on top. Shake until rice rises to the top. How long does it take?

* Conclusion: Compare the results. How did the trials differ? Why?

* Don't Stop Now!

See what happens when you run a trial with Ping-Pong balls and golf balls, which are nearly the same size but have different weights. Use a large plastic container instead of a jar.

Did You Know?

* Of all forms of electromagnetic energy, radio waves have the longest wavelength: from 104 to 101 meters long. Gamma rays are shortest: [10.sup.-11] to [10.sup.-14] m. The wavelength of visible light: [10.sup.-6] to [10.sup.-7] m.

* Physicist Paul Quinn hopes his BNP research will help geologists predict earthquake motion. Says Quinn: "The ground is a bunch of grains packed together. When earth shakes, at some point the big rocks rise to the top."

* Ketchup thins when shaken or stirred. A thick mixture of cornstarch and water displays the opposite behavior: Undisturbed, the mixture is runny, but when you stir it the mixture grows more viscous.

Cross-Curricular Connection

Social Studies: Starch is a dietary staple. However, different cultures tend to prefer different starches in their cuisines. Italians, for example, consume pasta made of wheat flour. Find out the most commonly eaten starches in Mexico, China, Ireland, India, France, and Kenya.

Critical Thinking:

Why do you think a paper towel is more absorbent than a sheet of typing paper?

Directions: Answer the following in Complete sentences.

1. What type of energy wave do humans see? And what colors are its longest and shortest wavelengths?

2. What are pseudoplastic liquids? Give two examples. (Define the following terms in your answer: viscosity, yield stress.)

3. A nut will sink or swim in a bowl depending on the ratio of what two properties?

Guess Who's Coming To Dinner?

1. Visible light is the energy wave that humans see. Red is the longest wavelength and violet is the shortest.

2. Pseudoplastic liquids behave like a solid until enough force acts on them. Then they flow like a liquid. Ketchup, toothpaste, and whipped cream are some pseudoplastic liquids. Yield stress is the minimum amount of stress needed to change the liquid's shape. And the viscosity or thickness of ketchup depends on the amount of stress to which it's subjected.

3. A nut will sink or swim in a bowl depending on the ratio of its mass and diameter.

Resources

How to Dunk a Doughnut by Len Fisher, Weidenfeld & Nicolson, 2002

How Things Work: The Physics of Everyday Life by Lou Bloomfield, John Wiley & Sons, 2002

For physics explanations in plain English, visit the American Physical Society's Physics Central Web site: www.physicscentral.com.