Friction opposes motion between two surfaces but facilitates motion within certain situations. It becomes smaller when lubricant is applied between surfaces. Friction makes movement difficult when it causes surfaces to become rougher. It can take the forms of static, sliding or rolling friction and is always present whenever two surfaces are in motion.

1. A RIDDLE FOR YOU
In some situations, I oppose the motion
In other situations, I facilitate the motion
But, I always oppose
The relative motion
Between two moving surfaces.
Put some lubricant, and
I become small there.
Making the moving surfaces rough
I make the movement tough.
I may be –
Static, sliding, rolling
But whenever two surfaces are in motion
I am always there,
Tell me who I am!
Answer: Friction