What crunches when I bend my knees, especially while exercising? Perhaps the best explanation for this phenomenon is a theory related to air pressure and nitrogen. All of your joints contain air and fluid. Nitrogen is a major component of both air and joint fluid. When you stand up (or make other movements), the pressure in the joint changes and nitrogen forms tiny bubbles. At times, the earth's gravity is enough for these bubbles to form. The crunch is the sound of the bubble moving. For instance, when you get out of bed in the morning, where you've been lying all night, nitrogen and air move, reacting to gravity.