A free jet of air entrains molecules of air from its immediate surroundings causing an axisymmetrical "tube" or "sleeve" of low pressure around the jet (see Diagram 1). The resultant forces from this low pressure tube end up balancing any perpendicular flow instability, which stabilises the jet in a straight line. However, if a solid surface is placed close, and approximately parallel to the jet (Diagram 2), then the entrainment (and therefore removal) of air from between the solid surface and the jet causes a reduction in air pressure on that side of the jet that cannot be balanced as rapidly as the low pressure region on the "open" side of the jet. The pressure difference across the jet causes the jet to deviate towards the nearby surface, and then to adhere to it (Diagram 3). The jet adheres even better to curved surfaces (Diagram 4), because each (infinitesimally small) incremental change in direction of the surface brings about the effects described for the initial bending of the jet towards the surface. If the surface is not too sharply curved, the jet can, under the right circumstances, adhere to the surface even after flowing 180° around a cylindrically curved surface, and thus travel in a direction opposite to its initial direction. The forces that cause these changes in the direction of flow of the jet cause an equal and opposite force on the surface along which the jet flows. These Coandă effect induced forces can be harnessed to cause lift and other forms of motion, depending on the orientation of the jet and the surface to which the jet adheres. A small surface "lip" at the point where the jet starts to flow over that surface (Diagram 5) increases the initial deviation of the jet flow direction. This results from the fact that a low pressure vortex forms behind the lip, promoting the dip towards the surface.