This is the best way to react to a knee slapper joke.
Step 1: EXPOSE YOUR KNEE
Don't be afraid to show your skin.
Step 2: FULL OPEN PALM SLAP
Like you would give someone if they did something.
Step 3: CONTACT
This must be fast and loud. Only do this when a joke is so good it is worthy of a knee slap.
Step 4: Video
I don't know how to put a video so I just put this. You can look up(stupid knee slap)