If you've ever laced your fingers together, turned your palms away from you and bent your fingers back, you know what knuckle cracking sounds like.
 crack knuckles crack joints pop knuckles pop joints就是把一手掌推另一手握拳的指頭而發出咯咯的聲音
這是一個連環問題: 燃燒石油導致二氧化碳增加，二氧化碳增加導致溫室效應，溫室效應導致冰層融化、海平面上升。 (請填空)
This is a chain of problems. Burning oil leads to increase in carbon dioxide, which leads to the greenhouse ffect, which leads to melting of glaciers, which leads to rising sea levels.