How do toy building bricks hold together?

A toy tower can stay in one piece when you pick it up. Its bricks are not glued together. Small bumps on one brick fit tightly into spaces under the next brick. The close fit helps the pieces hold on.

The bumps are called studs. In one common kind of brick, the hollow underside has tubes inside it. A stud fits between a tube and the outer wall. The plastic bends a tiny amount when the pieces join. You cannot easily see this little bend, but it helps the surfaces press together.

The pressing surfaces grip. This grip makes the bricks harder to pull apart than two smooth blocks resting on each other. A tug can still separate them. The same pieces can become a tower today and a little bus tomorrow.

The spaces must be made carefully. Too much room would make the join loose. Too little room would make it hard to push the pieces together. A useful toy brick needs to hold on and let go. Its hidden underside helps it do both.