Bellum eats life force from all things around it, starting with the Ocean King. This would also explain why the Ghost Ship took Tetra away at the start, as a means of bringing food to its master. But when you think about Skyward Sword, it makes much more sense than expected. If a creature consumes life energy, what more delectable source exists than a goddess?
At the end of the game, Tetra asks how long they've been gone for, and her crew says that it's only been "about 10 minutes". Why 10 minutes? That was initially the same amount of time the Hourglass could protect Link from the Temple of the Ocean King's curse.
The way that one would get the crest for the Sun Key [note] Pressing the 2 screens together [/note] is brilliant because many players might get frustrated and close their DS to look up the answer, thus solving the puzzle.
Which is also carried over into the Virtual Console port for the Wii U, in which "open and close the menu" substitutes for "close and then open the DS" - many players, upon being stumped by this puzzle, would open the menu to go onto Miiverse and ask for help...only to return to the game and discover that they solved the puzzle without knowing it.