Have a youth stand on a stool and proceed to push them off without them expecting it! They were not prepared, so they fell right off.
Ask them to get back on and then try to push them off again. This time (usually) they don't fall off but they are still quite "moveable" now give them a "shield of faith" and a "sword of truth" to help them fend you off.
Now not only are they immovable, but you can't even get near them! All these things help us to be "Steadfast and immovable."