Tap any tube to pick up the top ball, then move it to another tube. Important rule: you can only place a ball on top of another ball of the same color or into an empty tube. If you get stuck, don't worry! Use the "Undo" button or add an extra tube to help you solve the puzzle.
On PC: Use your mouse. Click a tube once to pick up the top ball (it will lift up). Click another tube to drop it there. On Mobile: Use touch controls. Tap a tube to select the ball, then tap the target tube to move it.