HEISENBERG UNCERTAINTY
How uncertain is uncertainty? So much so that, according the Heisenberg uncertainty principle, you can't measure everything you'd like to. Particles have certain pairs of properties (such as momentum/location, or spin/angular position, or energy/time) that are ``complementary.'' In practice, that means you can't make accurate measurements of both. The more you know of one, the less you can know of the other.
Classical physics had a lot of uncertainty, too--lots of measurements weren't particularly accurate. But physicists assumed the problem lay in the imprecision of the instruments--and that some day, with better instruments, you'd get more precise data. Instead, the Heisenberg principle says that uncertainty is not a mistake but a law of nature.
