What is a marine?

At the park today, the kids were intriuged when a group of marines stopped from their morning jog and began to do exercises on the playground equiptment. They were in the middle of doing chin ups, pull ups, etc. when one young boy exclaimed to his friend "those are the strong boys who like to eat hot dogs!". The school had a yard sale the previous weekend and apparently lots of these strong boys came by for hot dogs.