You lose if you draw a line that exits from the top or bottom of the screen, so it's important to avoid that when possible. However, the left and right horizontal edges are a different matter, an in fact you can use them as part of your strategy. Since a line will generally keep going until it exits along the top or bottom of the screen (or runs into a colored dot), you can let it rebound from the edge of the screen and it could keep going to help you solve the puzzle.