Atom's opposite.

Scientists in Europe recently created the first human-made antiatom, an atom with all the usual electrical charges reversed.

The scientists created antihydrogen because of its simple structure, explains John Eades, coordinator of the experiments. Regular hydrogen atoms have one negatively charged electron orbiting one positively charged proton. All the scientists had to do was make one positively charged electron orbit one negatively charged proton.

But those oppositely charged subatomic particles don't exist in nature, either. So first, the scientists had to create negatively charged antiprotons in a particle accelerator. Next, they shot a beam of antiprotons into xenon gas to create antielectrons. In at least one case, an antielectron began orbiting an antiproton and created - briefly - antihydrogen.

How do the scientists know? Because when antiatoms meet up with regular atoms, they crash and destroy one another, leaving behind a unique trail of subatomic debris.

The crash also gives off lots of energy - so much that some scientists believe antiatoms could one day be used as a lightweight rocket fuel for interstellar space travel. (That's how the fictional spaceship Star Trek Voyager gets around.)

But don't hold your breath. Creating a single antiatom was difficult - and for now, scientists say, making the millions needed for rocket fuel is impossible.