"To every action there is an equal and opposite reaction". I've done a very scientific test of Newton's law and it seems that, yup, he's right.
This past week my lower belly felt weird and I took my took hands and pushed slightly upward on it. A strange and immediate quick poke of pain where "Baby A" is located made me jerk. Low and behold, one of these little boys gave me a swift kick back in response as if saying "hey, hands off lady". It was strange --- and amazing.