why don't we have back buttons too?

E likes to climb on my back when I get down on the floor in child's pose (hunched over with legs tucked under your body). It feels great to me, like a mini-massage. When she was getting ready to climb on yesterday she suddenly lifted my shirt and seemed to be looking for something on my back. She said, with her usual urgency, "button!" Then she lifted her own shirt and looked for her belly button.