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Take your thumb and put it just above the joint(towards the top of your finger)

Now take your pointer finger and rest it under the first joint of your thumb. The tip of your pointer finger should just about be at the webbing between your thumb and other pointer finger. You should now have a finger on either side of the "thumb bone" and on opposite ends.

Gently apply pressure with your holding thumb downwards. Not the thumb that is being held. It should have about a half centimeter of wiggle room before it pops.

You can also pop the last joint in your thumb by cradling it similarly, titling it 90° from the rest of the thumb, and pulling away from the hand. This technique works on every finger tip.