Loaf of Bread

The housewife answered a knock on the door and found a total stranger standing on the doorstep.
"Excuse me for disturbing you, ma'am," he said politely, "but I pass your house every morning on my way to work, and I've noticed that every day you appear to be hitting your son on the head with a loaf of bread."

- That's right.

- Every day you hit him on the head with a loaf of bread, and yet this morning you were beating him with a chocolate cake.

- Well, today is his birthday.