NASA's Uranus Probe

Why do you have to watch your back at NASA? They want to probe Uranus.

Explanation

This is a classic pun using the double meaning of the word 'probe'. On one hand, NASA uses probes to explore space, and on the other 'probe' can be used as a vulgar verb. The punchline implies that people at NASA want to 'probe Uranus' in the crude sense, making it a humorous innuendo.