Standing without help - or anything to pull up on!

John and I were running around this morning, getting ready to go to work and had plopped JJ in the pack n play (playpen) while we took care of a few things.  He's playing quietly, sort of talking to himself and all of a sudden it gets really quiet.  John walks in and I here "OhmyLord!"  I come running in and JJ is standing there in his playpen smiling at us.  Standing.  Now, there's NOTHING in the playpen for him to grap onto - it's a mesh side affair with no bars or anything like that inside.  So how the #@% did he get standing up?!?!  Yikes!  John is going to have to get moving on the childproofing!