The authorities instruct: If You See Something, Say Something. That's how they found the car bomb, you know! Of course, this is hardly practical. You see something every day in New York. When to actually say something? A guide, below.

If you see: Vehicles parked haphazardly in Times Square with smoke emanating from them, say something.
If you see: Vehicles parked haphazardly in a non-tourist area with smoke emanating from them, make sure you're not parked next to them.

If you see: A Muslim man with a long beard standing on a street corner screaming "Death to America," and that man is in fact Osama bin Laden, say something.
If you see: Any other Muslim man with a long beard standing on a street corner screaming "Death to America," it's just those crazy shouty dudes.

If you see: An angry person pull out a knife and stab a fellow subway passenger, say something.
If you see: An angry person pull out a knife and nobody has been stabbed just yet, run.

If you see: A sweaty, suspicious, nervous character on the subway reaching into his backpack, which contains a homemade bomb, say something.
If you see: A sweaty, suspicious, nervous character on the subway reaching into his backpack, that's me coming home from work, GOD why are all you people TOUCHING me?

If you see: An unattended bag left in a crowded area, and when you peek inside that bag it is full of shrapnel and a timer and lots of wires and shit, say something.
If you see: An unattended bag left somewhere, do you know how many bags there are in this fucking city?

If you see: A plane flying alarmingly low over Manhattan, say something.
If you see: A plane flying alarmingly low over Queens, you live in Queens.

If you see: Several men piling machine guns into the trunk of a car while loudly discussing how they are about to use those machine guns to carry out a deadly attack on a crowded civilian target in New York City, say something.
If you see: Several men piling machine guns into the trunk of a car, that's gangsta.

[By the time we realized that Slate did something like this already today, it was too late. Pic via.]