( ( When ( ( two surfaces ) ( try ( to ( ( ( move ( past ( each ( other ( ( ( these little ) bumps ) collide ) ) ) ) ) and ) ( ( slow ( ( the motion ) ( of ( the surfaces ) ) ) ) ( down ( causing what ) ) ) ) ) ) ) ) ( we ( ( call friction ) . ) ) )
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (CD two) (NNS surfaces)) (VP (VBP try) (S (VP (TO to) (VP (VP (VB move) (PP (IN past) (NP (DT each) (JJ other) (ADJP (NP (DT these) (RB little)) (JJR bumps)) (NN collide)))) (CC and) (VP (VB slow) (NP (NP (DT the) (NN motion)) (PP (IN of) (NP (DT the) (NNS surfaces)))) (PP (IN down) (S (VP (VBG causing) (NP (WP what)))))))))))) (NP (PRP we)) (VP (VBP call) (NP (NN friction))) (. .)))
When two surfaces  try to move past each other these little bumps collide and slow the  motion of the surfaces down causing what we call friction.
(ROOT (S (NP (NN Friction) (NN force)) (VP (VBZ is) (VP (VBN caused) (PP (IN by) (NP (NP (JJ microscopic) (NNS bumps)) (, ,) (VP (VBZ lumps)) (, ,) (CC and) (NP (NP (NNS imperfections)) (VP (VBG colliding))))))) (. .)))