How do you explain these things... ?

Daughter and grandkids came over to eat grilled chicken and hang out last night. I had set up a little pool on my deck this past weekend for the grandkids, and filled it with water in hopes it would at least warm up a little from all the sunshine we have had this week. This was the first time they had been to my house since I put the pool up. BT said the water was still too cold to get in. BK did not share those feelings. She wanted to know where her swimsuit was. I have a drawer where I keep clothes for them, and I told her her suit was in the drawer and she went and got it, raved over how cute it was (it was cute), and asked for assistance in getting it on. BT saw his swimsuit, deemed it the coolest swimsuit EVER, and decided to give it a go since he did not want to be outdone by a 2 year old. BK is fearless, and was immediately doing the "cannonball" every 5 seconds. She just takes a diving jump, yells "cannonball" and lands flat on her belly in the water. BT was a little more reserved since in his opinion the water was cold. After a little while, BK decided to take her swimsuit top off. When Daughter realized she was naked from the waist up, she told her to put her top back on. BK promptly told her BT didn't have his top on.

How do you explain these things?

BT proceeded to tell his sister he was a boy and boys could go without shirts. She disagreed and remained topless.