Dirty Trick

There was a family gathering, with all generations around the table.

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Mischievous teenagers put a Viagra tablet into Grandpa's drink, and after a while, Grandpa excused himself because he had to go to the bathroom.
When he returned, however, his trousers are wet all over.  "What happened, Grandpa?"  asked his concerned children.

"Well," he answered, "I don't really know. I had to go to the bathroom; so, I took it out and started to pee, but then, I saw that it wasn't mine, so I put it back!"