Suspicious Kid

A mother and her four year old son were putting out cookies for Santa on Christmas Eve. Accidentally, the mom dropped one cookie on the floor.
“No problem,” she said, picking it up and dusting it off before placing it back on the plate.
“You can’t do that,” argued the child. “It’s now got bacteria on it.”
“Don’t worry,” replied the mom with a sneaky smile, “Santa will never know.”
The child thought about it for a moment and said, with a note of suspicion in his voice, “So he knows if I’ve been bad or good all year, but he doesn’t know the cookie fell on the floor?”
Busted!