The Letter 'M'

The teacher, Miss Jones, had been giving her second-grade students a lesson on science. She had explained about magnets, but didn't tell them what it was. She showed how they would pick up nails and other bits of iron.

Now it was question time, and she asked, "My name begins with the letter 'M' and I pick up things. What am I?"

Little Johnny in the front row proudly said, "You're a mother!"