Happy Cup is a physics-based logic-puzzle game with a drawing element similar to the Sugar Sugar series. With this game, the player helps direct the flow of clear, blue water from a large water bottle into a clear, empty anthropomorphic cup. The cup experiences happiness when it becomes full of water up to or over a dashed horizontal fill line near its invisible rim.
The player draws pink lines on the screen to create channels to direct the water. Each of the game's levels features obstacles that can cause the water to flow below the cup's position or off the sides of the screen without the player's intervention. These obstacles include cylinders, planks, shelves, walls and inclines of varying sizes and shapes.
Happy Cup requires that the player have a solid grasp of basic physics, especially real-world ideas about gravity. They also need to have a steady hand to draw lines with precision. When they perform this task correctly, tiny stars for confetti appear above the cup and at the end of the level.