Q: What is the difference between here and there?

Q: What is the difference between here and there?
A: The Letter T.

This is a riddle that uses the spelling of the words here and there to make a joke. The person hearing the setup of the joke is expected to understand it as meaning "what is the difference between our present location and another location that is far away from this location?", when the jokester instead answers the question "What is the difference between the word here and the word there?"

The answer to the second question would be obvious if it were worded in that way, however, the first parsing is more likely to be the parsing that people who don't know the joke would come up with.

The temporary confusion of hearing the answer, and then figuring out the true question is the root of the humor of this joke.