meaning: You are not fooling me

Question

This is from Good Luck Charlie.

Teddy, Gabe and PJ came to a restaurant to see their parents to tell them Charlie was lost. But suddenly they saw Charlie was in the restaurant. They were very excited. At this moment they did not know what to do. Should they still tell their parents what had happened to Charlie, or should one of them talk with them, distracting them while others went to fetch Charlie?

Teddy went up to mom and said,

"Hey mom. We just realized you must be missing Charlie, so we brought her. "

Mom said, "Teddy, you're not fooling me."

Teddy said, "I'm not?"

Mom said, "Nope. You spent a little too much time with Charlie, so you are ready to hand her over to me earlier."

I can understand "You're fooling me": you're trying to deceive me, you're telling a lie. But I can't understand the negative one: "You're not fooling me": you're not telling a lie, you're not trying to mislead me, you're telling the truth.

But mom later said "you spent a little too much time with Charlie, so you're ready to hand her over to me earlier."

By saying this, mom negated what Teddy had said earlier, "We brought her because we realized you must be missing Charlie". She actually meant "you're fooling me".

So what did mom mean by saying "you're not fooling me"?

Answer 1

You're not fooling me.

Usually, in this phrase, there is an emphasis on me. Depending on the context, it can have different shades of meaning.

• You can fool anybody but me.
• Whatever you are saying to me, I know it's not true, and I don't believe you.
• I know that you are telling me a lie.
• I am not letting you fool me.

You can also look up a similar expression:

You're not fooling anyone.

which means the lies you are telling are so apparent that no one (including me) would believe them.

Answer 2

Specifically in this exchange, Teddy tells his mother that he brought Charlie back early because Mom must be missing her. In other words, he's pretending that he is putting his mother's interests ahead of his own. Mom is saying that she sees through Teddy's deception (you're trying to fool me, Teddy, but you are not succeeding), and "knows" that the only reason that Teddy brought Charlie back early is that he is tired of watching her.

The humorous part about it is that the real reason that they came early was that they had lost track of Charlie, and when they found her in the restaurant, their "cover story" was that they brought her back because Mom probably missed her. Mom "saw through" their lie, but still came up with a "real" reason that was completely wrong.

So, the next part would be for Teddy to let his face fall a bit, and say something like "Mom, I shouldn't even bother to try and fool you. You see through me every time." Which of course is not true, but it helps to hide the fact that they lost Charlie when they were supposed to keep an eye on her.