I was talking to one of my children about units of speed. We'd covered miles per hour, kilometres per hour, and proportions of the speed of sound. Then I said, "How about some more units - you need a unit of length and a unit of time."
She suggested centimetres per hour, so I asked what might move at centimetres per hour, and she suggested a snail.
"OK, another unit of speed, then."
"Millimetres per hour."
"And what might move at that sort of speed?"
"A snail with a broken leg?"
