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The equal transit time theory is an incorrect theory about how lift is produced. According to the theory, the air going above a wing has a longer path to travel due to the curved shape, so the air molecules going over the top must travel faster in order to meet the air molecules that went underneath the wing at the same time. This theory is not true because there is no reason for the air traveling above the wing to meet up with the air on bottom. In fact, the air above the wing gets to the end of the wing faster than the air on the bottom of the wing. You can see this by watching the air above and below the wing in our How Wings Work activity.