This one is going to be a real shortie:
Q: Why does a dog wag its tail?
A: Because the dog is smarter than its tail.
If the tail would be smarter, it would have wagged the dog.
Now, this works on so many levels, I just cannot begin to list them all down. Those interested, there’s a movie by the same name. Go check it out. Plus, Mark Knopfler did the music for it, so it’s pretty nice.