Have a look at the picture
My child sometimes plays with pens, which could be dangerous because the ballpoint tip of the pen may poke him in the eyes.
How to idiomatically express "to press the end cap to make the ballpoint tip of a pen stick in" in simple terms to a child? (which looks like the picture below)
stick 4 MOVE PART OF BODY [transitive always + adverb/preposition] if you stick a part of your body somewhere, you put it in a position where other people can see it SYN put
Clara stuck her head around the door to see who was there.
The baby stuck his legs in the air.
Don’t stick your tongue out. It’s rude!