What is my 5 week old doing?

My son is 3 1/2 months old now, but when he was about 3-4 weeks old he would always do that. He started on my fiancees stomach and would scoot himself all the way up to his neck because that was where he wanted to be. he got good at it and by 5-6 weeks he could move himself from one side of the bed to the other, and then kinda plateaued he still does the same thing. its so cute (like a little caterpillar or something!) and its funny cause nobody would believe me when i told them. I guess some babies just develop faster than others. LUCKY!!!