Cool.<p>It's impossible using an unmarked straight-edge and pair of compasses to trisect a general angle.<p>It's impossible using an unmarked straight-edge and pair of compasses to compute the cube root of 2.<p>It's impossible using an unmarked straight-edge and pair of compasses to construct two lines whose lengths are in the ratio 1:pi.<p>It's impossible to show that given Euclid's first four postulates, the fifth must be true.<p>It's impossible to show that there is no infinity between aleph_0 and 2^aleph_0.<p>It's impossible to create a Turing machine that can predict if a general Turing machine will halt in finite time.<p>Do you want some more?