(I work in a clothing clearance outlet and I’m helping a kid pick out a pair of shoes whilst his mother is on the phone.)

Customer: “How much are these shoes?”

Me: “$100, but we’re having a sale with 50% off the price of
everything in store, so those will end up being $50.”

Customer: “So they’re free?”

Me: “No, they’re $50.”

Customer: “But 50% off $50 equals nothing, so they’re free!”

Me: “No, the original price is $100. It’s 50% off that price, which ends up being $50.”

Customer: “Hey mum, these shoes are free!”