What: Joining two pieces tightly so they hold without relying on adhesive alone.
Why: Structural integrity is the craft's base; a tight joint lasts, a loose one comes apart.
How: Measure precisely, cut cleanly, and dry-fit before fixing; the line of light should disappear.
Common mistake: Filling a joint's gaps with glue instead of a tight cut.
Exercise: Make a simple joint and test it: lift it from one end and confirm it holds.