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Hydrogen fuel cell cars, whose only tailpipe emission is water, are often hailed as one solution to our energy woes. But many hurdles remain, including lowering the cells' cost, building filling stations, and finding green ways to produce hydrogen. Meantime, our NOVA scienceNOW host test-drives a prototype fuel cell car, with "Car Talk" hosts Tom and Ray Magliozzi along for an amusing ride.

All About G ForcesWhat's behind gravity forces, and how much of them can we take? By Peter TysonPosted 11.01.07NOVAA few summers ago I took my then nine-year-old daughter on a glider ride. Midway through, as we soared over a coastal landscape, I casually asked the pilot whether he could do any tricks. Without a word, he threw the plane into a dive. We were accelerating straight towards the ground. My daughter and I shouted and grabbed the armrests. Suddenly we were hit with that thrill-inducing pressure familiar from rollercoasters-tensed facial muscles, light-headedness, a sense of altered reality.