[tag=NN] [lemma=","] [word=they] [lemma=□] □
[tag=NN] [lemma=","] [word=they] [lemma=explore] □
[tag=NN] [lemma=","] [word=they] [lemma=","] □
For instance , they explored the collisions of circular waves similar to those created by drops of water falling into a puddle .