Photo: surprise, it's filled with mayonnaise!

おにぎり. The "peanut butter and jelly sandwich" of Japan, if you will. You'll find these in most lunch boxes and everyone knows their mother makes them the best. Pictured here is the 7-11 version of onigiri and as hard as I try I cannot seem to make them myself with the same taste. Of course, this is probably analogous to someone trying to recreate a McDonald's hamburger.

What's inside the rice and seaweed is anyone's guess. Much easier if you can read Japanese, though.