As I placed the shell next to him, he would ignore it and immediately return to the smaller shell! After another 20 minutes of patiently watching him strain to get into the smaller one, I grabbed him, and stuck his rear end into the larger shell. I don’t think he wanted to admit defeat because he just sat there, motionless. After about 15 minutes, he got the idea, accepted the gift, and sauntered off with a new "butt home".