Lowering the crib mattress

We decided that Charlie is getting too close to pulling himself up that he is no longer safe with the mattress being at the highest level.  So, today, with the help of Zachary and Nicolas, Steve lowered the crib mattress.  We put Charlie in and let him stand up.  He was so proud of himself.  I get the feeling that if I let him do that a few more times, it won't be long before he starts pulling himself up.