Tony once got the idea to create a lightweight replica of Mjolnir. He would then nonchalantly carry it around for the day, seeing how many people he could fool into thinking he of all people was worthy of lifting the hammer.
Hardly above a harmless prank, Thor agreed to help Tony with the endeavor, but then decided to play a prank of his own, switching the replica with the real thing while Tony was distracted.
Tony never noticed, and Thor has yet to tell.
Submitted by anon