Mom: I'm going to the shop. If someone is at the door, don't open it.
Me: Ok.
*Ring*
Me: Opens the door.
Oh sh*t!
Mom: Gets flip flop.
Mom: I'm going to the shop. If someone is at the door, don't open it.
Me: Ok.
*Ring*
Me: Opens the door.
Oh sh*t!
Mom: Gets flip flop.
Listen up, buttercup. The joke is about a kid who was told not to open the door for anyone while their mom was out. But when the door rang, the kid forgot and opened it, only to realize it was their mom who told them not to open the door in the first place. The "flip flop" is a threat of corporal punishment, so the mom is going to hit the child for disobedience.