Now consider a bike brake.

The rubber pads rub against the wheel, so friction slows the bike.

The rider wants to slow down, so friction is helpful.

The rider wants to slow down, so friction is helpful.

But now consider a door hinge that squeaks and sticks.

You want the hinge to swing freely, so friction is unhelpful.

A drop of oil makes the hinge smoother, so there is less friction and it swings again.

You want the hinge to swing freely, so friction is unhelpful.

Now Reed's dad strikes a match.

The match head rubs against the box, and the rubbing warms it until it lights.

He wants the match to light, so friction is helpful.

He wants the match to light, so friction is helpful.

But now consider a drawer that sticks and will not slide open.

You want the drawer to slide, so friction is unhelpful.

You want the drawer to slide, so friction is unhelpful.