Cavitation occurs when the local pressure in a liquid drops below the vapor pressure of the liquid, causing the liquid to boil. A common situation where this can occur is in flow around propeller blades. As a liquid is accelerated over propeller blades, the fluid pressure decreases in response to the increase in fluid velocity (according to a relationship known as Bernoulliís principle). If the flow reaches a high enough velocity, the pressure can be reduced enough to cause cavitation. Bubbles that form as a result of cavitation have a much lower density than the surrounding liquid and the ability of the propeller to generate thrust suffers. Additionally, when the bubbles move to regions of higher fluid pressure, they will collapse and the force of collapse can damage the propeller surface.