Ever since then I've been pondering how I could have answered him, and I may have come up with a good metaphor. You can even do a demonstration!
Take a mostly empty ketchup bottle and then squeeze it FOR A WEEK STRAIGHT.
Bonus: I am both the ketchup bottle and your poor cramping arm muscles. Think about that. THINK ABOUT IT. How do you think that ketchup bottle feels?</i>
Cramp-having people, any ideas for fun/useful explanations of cramps for the curious?