A dentist was examining a patient and said: "I've got to be honest, your teeth are very clean but they are the most badly chipped and damaged set I've seen.
What on earth are you doing to cause this?"
"Well, I'm a nervous guy and I do a lot carpentry work" said the man sheepishly.
"I don't understand," the dentist said.
"Well, I can't seem to stop biting all my nails."