Grandma and Grandpa were visiting their kids overnight. When Grandpa found a bottle of Viagra in his son's medicine cabinet, he wanted to try one of the pills.

His son said, “I don't think you should take one, Dad, they're very strong and very
expensive.”

“How much?” asked Grandpa.

“$10 a pill,” answered the son.

“I don't care," said Grandpa, “I'd still like to try one, and before we leave in
the morning I'll put the money under the pillow.”

The next morning, the son found $110 under the pillow.

He called Grandpa and said “I told you each pill was $10, not $110.”

“I know," said Grandpa. “The hundred is from Grandma.”