Probably by using vinegar, an acid that will dissolve the calcium in bones.
Calcium, as you probably know from school or television commercials, is what makes bones hard and strong.
Try it yourself: take the drumstick of a chicken and clean off all of the meat. Place the bone in a jar of vinegar and leave it for a week.
Take it out, and you’ll have a rubber chicken bone you can use in your vaudeville act.