Who's pee IS this?

So, Kate just turned 2 years old. Because she is constantly mimicking her brother and sister (for better AND for worse...) she seems older at 2 years old than her older siblings did. She's close to potty training, so I've taken our little wooden potty chair that I bought before Ellie was even born (sniff sniff) and set it up in our bathroom. I encourage her to sit on it, get comfy on it, etc. But, so far, she is not that interested in doing anything there other than perusing the odd book, usually upside down.

I was getting the kids into the tub yesterday, and while her diaper was off, got Kate to sit on the potty. Then, she went into the tub.

Fast forward to this morning. I was getting out of the shower and noticed that there was a tiny bit of pee in the potty. Did Katey pee in it yesterday, and I somehow missed it? I started to get excited, thinking Kate peed in the potty. I yelled downstairs, "Hey guys, who peed in Kate's potty?"

"Whaaaat??!!..." everyone yelled back. I said, "There is pee in Kate's potty. Who's pee is this?"

"Oh", Bradley said, "That was me."

"Why did you pee in Kate's potty?"

"Because Mommy, I wanted to remember what it's like to be little."

He's five.