Bite down on the soft portion of the valve and suck at the same time. This will start the flow of water to your mouth. When you're finished drinking, the valve will self-seal when not in use. To further protect against accidental spilling, you can also rotate the (blue) body of the valve 90 degrees until it is positioned on the mark with an "X". This will shut off any flow of liquids. To take another sip, the valve will have to be positioned back to the mark with an "O."