Check yourself, fellow diners

I'm not usually comfortable pointing out that someone is being deliberately obtuse. Actually, yes, I am comfortable pointing that out, but not always to strangers. Today, I crossed that subtle, yet sometimes necessary, line to show someone that they were being stupid.
Cryptic storytelling alert: There are two women sitting at a booth across from mine today. One knocks her knife onto the floor in the aisle. She sees the knife on the floor, but chooses not to reach over and pick it up. She pretends to not notice the knife. I get stressed out. Every time someone walks by, I fear that they'll slip on the knife and crack their head on the floor. The fourth person walks by and narrowly misses the knife. I decide that I've had enough. I slide out of my booth, bend over and retrieve the knife. I set it on the table and say, "Here, I'll get this for you. I keep worrying that someone will slip on it." They both thank me, and the woman has the nuts to say to me, "It's so noisy in here that I didn't hear it fall." I smile, and in my head say, "Yeah, and you're a lazy liar."
More times than not, I allow these situations to go by without saying or doing anything. I've decided that I'm going to be a bit more proactive though. There's really nothing wrong with being that person, especially when it comes down to a potentially dangerous event. If though, for the rest of their lives, these two women know that even though they may not care, someone may notice that they don't care, and that compels them to act responsibly, I'll be happy.
Look at me! I love it when I can play "more conscientious than you." The opportunity so rarely presents itself.