What is the most stupid thing you told someone and they believed was true?

For me: At car mechanic school. We were in transmission, my friend and I just took an oil shower (first time opening an on-car transmission, didn't go well :P), we look at the student next to us, he was tugging on the front tire of his car to get it off.

We asked him if he drained the tire's oil (we pointed our oil-soaked clothes) and he actually believed us, he even asked the teacher for the tire's oil key.

It's been a year and the whole class still laugh about it.