When you start mowing your boy's hair.
Ba dum bum
Get it? Mow your boy's hair?
That's the punch line. But I can provide photographic evidence if you need a visual punch line too.
Now do you get it?
I didn't mow this son's hair. He let his friends mow it for him.
Which reminds me of an ancient Chinese proverb that goes a little somethin' somethin' like this:
Friends don't let friends mow drunk.
Words to live by.