Guys, How did you find out you became physically stronger?

I have a young brother, we used to play to push each other and it used to be fine, we stopped playing like that for years. One day I pushed him playing in the same way and when he pushed me back we both were like "wtf?" cuz he basically pushed me to the wall. Wasn't his fault, he felt bad, just that, he's suddenly way stronger and he hadn't noticed.
Guys, How did you find out you became physically stronger?
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