Sleepsack: for warmth and other things

Joshua has a couple of "sacks" for sleeping. We have put him in a fleece sleepsack almost every time he goes into his crib for several months now. A couple of times in the past week or two, I have let him sleep without it. I thought it was warm enough, but then he pulls off his socks while he is in his crib. Yesterday, it was so warm that he wasn't wearing socks. I laid him down for a nap in his pants an his onesie. When I got him up a while later, I nearly laughed out loud. Since he didn't have the hindrance of the sleepsack and he didn't have socks to grab, he decided to pull off his pants! David and I are still not sure how our uncoordinated little 5 1/2 month-old managed this feat!