Liam's joke

Liam tells a lot of jokes these days, and most of them are not funny at all. For example, why did the ice cream truck cross the road? to get some more ice cream. The other day, though, he made a joke that was actually funny.

He was learning about weather at school, and studies thermometers. When he got home, he began to tell us what he learned, and how the mercury went up when the temperature went up. Matt tried to correct him, telling him that mercury was not used in thermometers anymore, but Liam didn't believe him. He repeated that Miss Jill said it was mercury. We obviously were never going to be able to contradict Miss Jill. Matt persisted, though, and really made his point that mercury was no longer used in thermometers, until Liam paused in his protesting, got a sly look on his face, and said, "I think thermometers have Pluto in them!" before bursting out laughing. It was actually pretty funny, and not a bad pun!