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It sounds like your pet has luxating patellas. In layman's terms, this means wiggly kneecaps. The kneecap pops out of place and they flex the leg to pop it back in. Then they play as normal.
There are effective surgeries that can fix this but many dogs live a life time with this condition and never have it repaired.
You should have your vet examine the knees the next time you see him for a regular exam.
Let me know if you still have questions.