Rubik's Cube Versus Penis

What is the difference between a Rubik’s cube and a penis? I don’t know, but they both get harder the more you play with them.

Explanation

The joke uses a double meaning of the word 'harder'. In the context of a Rubik's cube, 'harder' refers to the increased difficulty of solving it the more you play with it. But in the context of a penis, 'harder' is an obvious reference to sexual arousal. This creates a humorous comparison.