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