NOVA.  The great apes are our nearest relatives in the animal world and for decades we've known that some of them can use simple tools and even be trained to communicate with us in sign language. They have some of the largest brains in proportion to body size of any animals, but just how smart are they? Nova explores the secret mental lives of apes and the crucial gap between ape intellects and our own. Every time we've defined a mental ability that we think is uniquely human - everything from culture to simple math - the great apes have it too. But now scientists are zeroing in on tantalizing hints of what that essential difference may be.  56 minutes. CC.