Where do they get this from?

I had to post what happened this morning before I forget. We walked Grant to the bus stop which is just around the corner from our house. He and Scottie were racing there. I rounded to corner to find them out of breath and stretching. Then I see Scottie get down and start doing push-ups. Of course Grant can't be out done so he starts to do backward push-ups. Cars are driving by while they are doing this. They stopped and just played around until the bus showed up. As it stopped, Grant got on the bus and Scottie gets down and starts to do push ups on the sidewalk in front of the bus and all the cars following the bus. Really? When I made him get up he said "Mom, why'd you do that?" As if what he was doing was totally normal. . . . I blame these traits on my husband. Can't say I'd see myself doing push ups in front of my big sisters bus.