A.T. Robertson Commentary Matthew 12:2

"But the Pharisees, when they saw it, said unto him, Behold, thy disciples do that which it is not lawful to do upon the sabbath." — Matthew 12:2 (ASV)

Thy disciples do (ο μαθητα σου ποιουσιν). These critics are now watching a chance and they jump at this violation of their Pharisaic rules for Sabbath observance. The disciples were plucking the heads of wheat which to the Pharisees was reaping and were rubbing them in their hands (Luke 6:1) which was threshing.