A Joke I first heard from Gabe Kotter

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 wallop him on the head with a loaf of bread, and yet this morning you were hitting him with a chocolate cake....?'
'Well, today is his birthday!'