Marble Sort — marble sorting on a conveyor belt
At the top of the screen, there are trays filled with colored balls. You tap a tray, and the balls drop through a funnel onto the conveyor belt. The belt carries them past the boxes below. When a ball passes a box of its color, it drops on its own into a free slot. Each box has three slots. A full box disappears, and the next one rises up from below in its place. A similar three-of-a-kind rule works in Pop Fruit, but there the fruits are collected into baskets.
When the belt runs out of space
There isn't much room on the belt. Balls that don't have a matching box below yet stay on the belt and take up space. If the belt fills up with those balls, the game is over. That's why order matters: it's better to open the tray with a color that has nowhere to land right now later.
The first levels are easy. Later on, there are more balls, and hidden colors appear. One thing helps: the balls move to the boxes from right to left, so the boxes on the right usually fill up first. Matching a color to the right spot is also part of Color Path Puzzle, but there you move the pieces across the board yourself to their color zones.