re: What's the simplest thing you've had to explain to a full grown adult?

How to load a dishwasher. One of my roommates tried to load a dishwasher. After the load was done, we had a dishwasher full of bowls, cups and plates all full of water, and a kitchen floor covered in suds. I taught him how to do it right and which detergent to use. He never touched the thing again.