The best jokes are the ones that require some thought after you’ve got the punchline. Even better are those that require thought and specific knowledge; the more subtle, the funnier I find it and the more I appreciate it humor.
This is one of those that require both.
Just in case you don’t get it, 10 is binary for 2. Binary is a base 2 system, so there are only two digits, zero and one. So counting to ten in binary goes something like: 1, 10, 11, 100, 101, 110, 111, 1000, 1001, 1010. Of course, if you have to have binary explained, the joke loses its impact, but trust me, it’s hilarious!
Incidentally, this is another of the fine t-shirts I found at thinkgeek.com.