1. Einstein showed that light travels in bundles called quanta, which are at
the heart of the light-emitting diode. When electrons in a semiconductor-based diode move from one side to another, they shift to a less excited state, releasing energy in the form of photons. Channel these, and you get a bright, long-lasting light source.

2. In 1917 Einstein demonstrated that when a photon comes into contact with an atom, it can trigger a chain-reaction release of additional photons from

If you want to go from scrawny to brawny in 30 days, there is no shortage of miracle shape-up programs. But as impressive as beefy pecs and triceps may look, they won't help you cite the evidence for Einstein's special theory of relativity, rattle off pi to the 20th decimal place, or liberate yourself from the mass delusion that a penny dropped from the Empire State Building will gather enough speed to kill a hapless pedestrian. We at PopSci believe the body part most worth stretching and toning—not to mention showing off—is the brain.

When H.G. Wells sent the hero of The Time Machine into what Wells called "futurity," it was on a grim 30-million-year round-trip to pretty much the end of Earth time, when the last, poorest excuses for life were flopping around like squid under a darkening sun. Wells wasn't the first writer to imagine time travel, but he advanced the idea that a machine, rather than an angel or a bonk on the head, could accomplish it, and he pushed his machine to the limit.