Little Johnny's classmates kept taking the mickey out of him for being stupid.

They offered him either a £1 coin or a 50 pence piece

One day, after Johnny took the 50p again, his teacher took him aside and said "Johnny, those boys are making fun of you, don't you know that a pound is worth more than a 50p, even though the 50p is bigger?"

Johnny grins and says "Of course I fookin do, but I've made £20 out of them so far". :)