What: Ensuring the piece bears its function: the joint holds, the vessel doesn't leak, the structure doesn't loosen.
Why: Beauty without structure fails in use; structural soundness is the craft's base before ornament.
How: Test the piece in practice under its intended load, and review every joint and weak point before finishing.
Common mistake: Relying on adhesive to compensate for a poor fit.
Exercise: Make a joint and test it: load it with weight and confirm it holds without relying on glue.