Why do dogs shake their legs when you rub their belly?

From Googling:

“Tickle a dog’s tummy, and you could get a knee-jerk reaction. There is a certain area on a dog’s body that is referred to as the “saddle region.” It vaguely consists of the back, sides and flanks of the pooch. Rubbing or scratching a dog in the saddle region (the exact spot varies according to the individual pet) will cause the hind leg to simulate a scratching motion. This is called the “scratch reflex.”

The dog’s spinal nerves pass all the way to his chest and abdomen, and stimulation of these nerves will cause a feeling of “itchiness” (similar to the irritation of a flea) somewhere on his body, causing the rear leg to involuntarily attempt to scratch it. The leg extends to different lengths or reaches to varying heights depending upon the area that is stimulated. Veterinarians who suspect spinal or neck damage in canines use the scratch reflex as a diagnostic tool, much like when doctors tap humans just below the kneecap to test nerve reactions.”

or It feels reallllly good

My 14 year old asked me the same question about a year ago. I told him, totally straight faced, that although dogs feel you tickle them, they are incapable of laughter. The leg shaking is a transfer of the ‘tickle’ energy. To whit, a dog’s shaking leg equals canine laughter. I let him off the hook a few months later when I heard him relating this to a friend of his in a very learned tone.

Because it tickles!

@Blondesjon
You could say “I don’t know, let’s do some research to find out,” instead of making up a bullshit answer.

To keep from peeing. (at least, that’s why mine shakes. )

kinky

@Vinifera7…I could’ve but…

A. My children are both National Honor Society Members that are well aware of how to do that themselves.

B. We play Balderdash in real life. It’s a party game…Google it :)

bless his heart

@Blondesjon
Well, good. You get +10 respect points. ^_^