When an airplane ascends to its cruising altitude, the cabin pressure decreases, causing the air inside the Grayl to expand. To prevent any leakage, it is recommended to use a One-Way Silicone Valve. This valve will reduce the likelihood of leaks from occurring. Alternatively, you can fly with your bottle either completely empty (without any water to leak) or completely full (without any air to expand). Once you have reached the cruising altitude, you can proceed to use the Grayl normally.