Make a path for the water to reach the glass by drawing lines. To stay in place, the line must be supported. Because there is a finite amount of water, you must make sure that enough water is poured into the glass to complete the level!
There is a limit to how long you can draw a line. In the bar at the top of the screen, you can see how much money you have left. The bar also has three portions that represent how many stars you'll receive in the end. The fewer drawings you make, the better.
Happy Glass keeps you thinking by introducing additional challenging components as you progress. To make the glass happy again, you'll have to apply your reasoning and creativity.