Grandma and Grandpa were visiting their kids overnight.

When Grandpa found a bottle of Viagra in his son's medicine cabinet, he asked about using one of the pills.
The 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."
Later 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!"