A swept wing typically angles backward from its root rather than forwards. Because wings are made as light as possible, they tend to flex under load. This aeroelasticity under aerodynamic load causes the tips to bend upwards in normal flight. Backwards sweep causes the tips to reduce their angle of attack as they bend, reducing their lift and limiting the effect. Forward sweep causes the tips to increase their angle of attack as they bend. This increases their lift causing further bending and hence yet more lift in a cycle which can cause a runaway structural failure. For this reason forward sweep is rare and the wing must be unusually rigid.