How I know it’s the fetus kicking

When you poke yourself in the arm, you feel it twice, at once—on your arm, and on your finger.

When someone else pokes you in the arm, you feel it once—on your arm.

When you have a muscle spasm or some other internal movement that comes from your own body, you feel it twice, at once—in your muscle and in the surrounding tissue, e.g. the underneath of your skin if it’s close to the surface.

When my baby kicks, I feel it once. A pushing out from within, caused by something internally-external.