You can do anything you set your mind on

We were (jokingly) trying to convince Grace & Isabel that they should
change Holden's diaper.

Aimee; I think you can do it.
Grace: I've never done it before.
Aimee: I think you can do anything you set your mind on.
Isabel: No, she can't.
Grace: Yes I can.
Isabel: Then do it.
Grace: I didn't set my mind on it.