Grafting is placing one plant onto another so the inserted plant grows off of the other. This is commonly used with fruit trees and cacti (especially the colored ones, the colored part usually can't survive on its own). Here I show how to graft one cactus onto another.
Note: This may not always work. Not all cacti may be compatible and sometimes by attempting grafting you may harm one or more of your cacti. You are solely responsible for damage and injury caused by this instructable.