In DropSum you release numbered balls into a grid. The aim is to make a line of these balls add up to 9, either horizontally or vertically. When your column or row totals nine, you score points and all the balls involved change color. They start out grey and advance to blue, yellow and then red. Once a red ball has been used to make a sum of nine it will explode and allow any balls above it to fill the gap. The falling numbers then have the potential to form scoring combinations with their new neighbours, which may burst other balls, and so the chain reaction progresses. Setting up large combos like this is the way to score serious points. The game ends when the whole grid is filled.
Destroy special octagon tiles and you'll release a special star. These stars are gathered up after a short while. The more stars gathered, the higher the bonus you'll receive.