Marines are some of the best trained, hardcore and tough SOBs there are. Trained and groomed to act fast and do insanely brave things have earned them a few jokes over time in friendly inter-service rivalries. One of these is the infamous 'crayons for Marines' joke, which infers that only insane people and infants who don't care about themselves often eat crayons. This is probably the only way a Non-Marine can make sense of a Marine's bravery. See, your Marine burst into laughter when you get them this excellent gag gift.