butt muscles

I walked into the kitchen last night and my girls were ramming their hips into each others butts and laughing hysterically....what?  I have learned not to ask too many questions so I just watched, pretty soon I realized what they were doing.  They were flexing their butt muscles and then ramming into each others butts to check out how tight their butts were...yeah...touching it with their hands would have been just ridiculous....hahahaha!