Iron palming what do you use to train?

[Asdasdas]

When training iron palm what do you smack? (hit for the training) ? i hear some use beans, if you dont have beans does it work to hit anything else? like wood? or anything you know of that works around the house? how come they dont hit like wood, carpet or anything like that around the house why do they specifically use beans or w/e. and what other iron body parts can you train yourself and how