system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do painkillers like Ibuprofen work?
gpt
There's two parts to this.  Well, probably a lot more than two, but we're ELI5ish here.

First, you eat the pill.  It gets broken down in your stomach and intestine, just like regular food.  Food has nutrients that get broken down into base chemicals, and absorbed into the bloodstream.  Ibuprophen, and similar drugs, break down into base chemicals just like food.  But these chemicals are special.  Let's call our chemical Mac.  Mac likes to bind to other enzymes in your bloodstream, including one we'll call Charlie.  Charlie likes to hang out with a special enzyme named Frank.  Frank and Charlie together are bad news, and cause inflammation.  But if Charlie is hanging out with Mac, Frank can't join the fun.   

Now, side track, what is inflammation?  It's when things swell up.  That doesn't necessarily hurt on it's own, but if there is nowhere for the swelling to go, because say, you've got a bone or some skin there, we get pain.

Back to the Charlie and Mac.  Now that they are hanging out, and Frank can't join the party, we don't get inflammation.  Without the inflammation, we don't have pain.  

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This is an incredibly high level and basic description, but the gist is, ibuprofen and other similar over-the-counter painkillers block the production of a chemical that ultimately leads to pain.  Less of the chemcial, less of the pain.