Sky diving is becoming a popular recreational and competitive activity, involving descending partly by parachute from an airplane or similar craft. Sky diving may be classified as an inherently dangerous activity and many operators of sky diving companies require customers to sign a waiver of liability to protect them against injuries suffered from customers' participation in sky diving. However, the sky diving company is responsible for equipment, such as the parachutes that it rents, which must be kept in good condition so that when the customer pulls the cord, the chute opens up. Former President George H.W. Bush made two parachute jumps in 2004 as part of events celebrating his 80th birthday.