Keep it under your hat

Whenever someone tells me to keep a secret, they tell me to keep it under my hat.
The problem is, there’s only so much room under my hat for everybody’s secrets.
I ask them if I can put it under someone else’s hat, and they tell me no. It needs to be my hat. They trust me and me alone.
Fine.
What if I get a bigger hat? Is that okay?
Yes, they say.
So I trade in my hat for a stovepipe hat.
The rest is history. I became President, and that’s when I really needed to keep secrets.