A neutron walks into a bar.
A neutron walks into a bar. "I'd like a beer," he says. The bartender promptly serves up a beer. "How much will that be?" asks the neutron. "For you," replies the bartender, "no charge."
This is a physics joke, but I will get to the physics after I explain the bartenders comment.
Sometimes bartenders tell a well liked customer that their drink will be free, and they do it by saying "For you, no charge". This happens in stories - I don't know of any instances of it happening in real life.
Now, one of the things that everyone might remember about neutrons is that they are not electromagnetically charged, so the bartenders answer to the neutron could also be referring to its state of charge, which is humorous because it makes the person who hears the joke feel superior because they know a science fact.