Forgot Grandma's Rule Against Pranks

Yesterday, I decided to surprise my grandkids with a silly prank. I hid their favorite toy in a place they wouldn't think of looking and couldn't reach without help. But as the day went on, they got more and more upset about losing it. By bedtime, I felt terrible for making them cry, but I still enjoyed seeing the look on their faces that morning when they thought it was gone forever. Am I a monster? Should I tell them what happened or let this be their mystery toy?