If you spend any time arguing with creationists (I used to do a fair amount of this), it's helpful to have some actual data on hand. I'm a math-geeky guy, so I like charts and stuff. One of my favorites is from an old post on Panda's Thumb.
The thing is, there's really an astonishing amount of evidence for human evolution: a dozen extinct hominid species comprising hundreds of fossils. Compare that to the number of chimpanzee fossils that have been found: zero! Some religious folk admit that the evidence for evolution is overwhelming, and accept that it was the way all animals and plants came about - except for humans, who were specially created by God. Given the fossil evidence, though, it would be much easier to make the case for special creation of chimps!
Brain size is, of course, not the only important difference between humans and other animals. But there are some profound advantages to focusing on a single characteristic like this. In the usual way of discussing evolution, one classifies the fossils into different species and talks about the average characteristics of each species, "Homo habilis had an average cranial capacity of 500 ml," etc. But then, creationists point to the "jumps" in brain size between species and shout, "Gaps in the fossil record!" The graph, on the contrary, shows that there really are no gaps in the brain size record. Likewise, when discussing one species at a time, creationsists will try to call them "just another ape" or else "just ancient humans." Show them this graph and ask them where they draw the line between apes and humans.
Finally, the graph refutes the creationist claim that the theory of evolution makes no testable predictions. The link between brain size and age in hominid fossils is a clear prediction of Darwin's theory: if humans evolved from smaller-brained primates, then there must have been primates with intermediate brain sizes. At the time Darwin made his proposal, there was no fossil evidence to back it up. (Only one Neanderthal fossil was known at the time, and no other ancient hominids.) The discovery over the last century of all these intermediate fossils provides resounding confirmation of the theory - regardless of whether these species were on the direct line of human ancestry or not.
(NB: the title of this post is stolen from this bumper sticker.)