Thanks, Beth and Elisabeth.
I think, and my memory of this is iffy, that Clark didn't start to really have powers (in LnC, anyway) until he was 10. So, Grandpa certainly didn't know Clark's secret in this story as Clark was only 8, so there was no secret to tell.
Still, it was more fun to do this with a grandparent than with Martha or Jonathan. Mainly as the rest of the scene was more fun to write with a "conspirator" - and that couldn't be one of his parents.
