(Mishnah): The Torah allows the following to eat the produce (while at work) - One who is working with finished food (meaning that there is no additional processing needed in the field) attached to the ground, unfinished food detached from the ground or anything that grows from the ground.

The following may not eat - one who is working with unfinished food attached to the ground, finished food that is detached from the ground or anything that does not grow in the ground (such as fish, eggs or milk). If he is working with his hands but not his feet or with his feet but not his hands, even if he is carrying on his shoulders, he may eat.

R. Yosi bei R. Yehudah: He must be using both his hands and his feet.

(Gemara) - R. Ila: The pasuk states (Devarim 23, 25), "When you shall come to the vineyard of your friend (you may eat grapes...)'' - did you expect him to eat wood and stones? Why does the pasuk mention grapes? To teach that you should not peel the figs and squeeze the grapes (whilst working as it would overly disrupt his work).