Because The Saying 'Where The Sun Don't Shine' Doesn't Apply When You're 3
Me: Why are you naked?
Her: Because I need to go to the potty.
Me: Why, then, are you just standing on the stairs?
Her: Because the toilet makes my bum cold...
(turns around to display cold bum)
... and there's a sunbeam here...
(bends over and waggles bare bum in the stream of sunlight pouring through the landing window)
... and it warms it up.
(pats her toasted bottom proudly, and then proceeds up the stairs to the bathroom.)
You just can't fault that logic.