( ( Think ( about ( ( the ( density difference ) ) ( of ( ( oil floating ) ( on water ) ) ) ) ) ) . )
(ROOT (S (VP (VB Think) (PP (IN about) (NP (NP (DT the) (NN density) (NN difference)) (PP (IN of) (NP (NP (NN oil) (VBG floating)) (PP (IN on) (NP (NN water)))))))) (. .)))
Think about the density difference of oil floating on water.
(ROOT (S (NP (NNP Density)) (VP (VBZ allows) (SBAR (S (NP (NN cut)) (VP (VBZ logs) (S (VP (TO to) (VP (VB float) (PP (IN on) (NP (NN water)))))))))) (. .)))