( ( ( A glass ) ( of water ) ) ( ( is ( used ( so ( that ( you ( can ( ( see ( by ( the ( air bubbles ) ) ) ) ( ( how constant ) ( ( your breathing ) is ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT A) (NN glass)) (PP (IN of) (NP (NN water)))) (VP (VBZ is) (VP (VBN used) (SBAR (RB so) (IN that) (S (NP (PRP you)) (VP (MD can) (VP (VB see) (PP (IN by) (NP (DT the) (NN air) (NNS bubbles))) (SBAR (WHADVP (WRB how) (ADJP (JJ constant))) (S (NP (PRP$ your) (NN breathing)) (VP (VBZ is)))))))))) (. .)))
A glass of water is used so that you can see by the air bubbles how constant your breathing is.
(ROOT (S (NP (DT An) (JJ underwater) (NN spider)) (VP (VBZ uses) (NP (JJ shiny) (NN air) (NN bubble) (S (VP (TO to) (VP (VP (VB breathe)) (CC and) (VP (VB keep) (ADJP (JJ alive)))))))) (. .)))