What did batman say to robin before they got in the batmobile?
Q: What did batman say to robin before they got in the batmobile?
A: Get in the batmobile.
This is anti-humor. The answer to the question, instead of being a normal punch line is something that Batman could reasonably have said to Robin (assuming that Batman was being a bit of a jerk to Robin).
Not being a huge Batman fan, I don't know for a fact whether Batman is in fact a jerk to Robin, however his persona seems to allow for that to be possible. It is also possible that part of the humor of this joke is that Batman is being a jerk.
This joke is similar to an elephant joke which I know of... perhaps this joke was even based on that joke.