What can be measured but not seen?

An old man who loves to fish, was sitting in his boat the other day when he heard a voice say, "Pick me up."
He looked around and couldn't see any one. He thought he was dreaming when he heard the voice say again, "Pick me up."
He looked in the water and there, floating on the top, was a frog.
The man said, "Are you talking to me?"
The frog said, "Yes, I'm talking to you. Pick me up. Then, kiss me and I'll turn into the most beautiful woman you have ever seen. I'll then give you more sexual pleasure that you ever could have dreamed of."
The man looked at the frog for a short time, reached over, picked it up carefully, and placed it in his front breast pocket.
Then the frog said, "What, are you nuts? Didn't you hear what I said? I said kiss me and I will give you sexual pleasures like you have never had."
He opened his pocket, looked at the frog and said, "Nah, at my age I'd rather have a talking frog."