Here is the answer I contributed to the question on Quora, where the answers were deeply inadequate:
Most of these other answers do a poor job of conveying the reality of what “illegal” means, in reality.
There are tons of laws that could be argued to apply to any given action. For instance, if you tell a police officer you had one drink when you really had three, it’s conceivable that that act in and of itself could be prosecuted as a crime in many states. But it basically never would be, because to police, it doesn’t really matter; they’re not really asking you if you drank tonight to gain information, but to hear how you answer, and to smell your breath.
On the other hand, if you called and falsely told the police that there was a police car on fire at some location and an officer bleeding in the street, and there had been a rash of false reports to the police in your area, then you very well might face prosecution.
What’s the difference? Is one act legal, and the other illegal? Well, you could read this law or that law and make a case for various interpretations, but in practice what matters is what the effect of your action is, and if police and prosecutors feel motivated to focus on it. Most states have enough laws on the books that significantly inconveniencing, harassing or impeding the work of police officers is something they can punish, on the basis of one law or another.
Note also that the most common punishment for crime is NOT prosecution. That is, most crime that gets punished in the US never goes before a judge, even as part of a plea deal. Most crimes that are punished are punished simply by arrest and release—police make someone stop and search them a bit, or make them sit in the car, or put them in handcuffs and take them to the station, maybe make them spend a night or a weekend in jail (arresting people on a Friday, so they’ll not be fully processed and released until Monday, is a common practice). Filing paperwork for someone is a hassle, and also sometimes police officers feel sympathetic to some people and don’t think an offense deserves prosecution. So, often police will just inconvenience the person. Often that person has indeed broken some official regulation; often they haven’t, or the police suspect that they did but don’t really know. So did they “do something illegal”? It’s not really easy to say, and that’s really not the right question.
Often there are no charges filed at all, or there are charges initially filed and then withdrawn. (Often, as well, the person’s belongings are stolen by police—they never make it to the itemization list when the person is processed, or they are listed but then come back with money or other contents removed. I’ve witnessed multiple people pleading with the NYPD at local stations for the return of their belongings taken this way, and they would have absolutely no reason to lie.)
It also matters a huge amount who you are, and how much attention the police expect your arrest and/or prosecution to attract. People are often rounded up with zero suspicion by the police that they have committed crimes, for instance at protests; in NYC, people have been thrown into paddy wagons while on their way to work, totally mystified at why they’re being arrested, only to be released hours or days later, never having been charged.
It’s simplistic, and essentially incorrect, to say “Yes, lying to police is a crime, it’s called _____________” or “No, lying to police is not a crime”. Smart lawyers never talk to each other that way! The idea that some things “Are Crimes” is a simple gloss that the unthinking public is told, but not at all how crime and punishment actually work.