The article by Ken White referenced in the sibling comment by likpok does a good job explaining it:
https://www.popehat.com/2011/12/01/reminder-oh-wont-you-plea....
> Here's how it works. The feds identify some fact that they can prove. It need not be inherently incriminating; it might be whether you were at a particular meeting, or whether you talked to someone about the existence of the investigation. They determine that they have irrefutable proof of this fact. Then, when they interview you, they ask you a question about the fact, hoping that you will lie. Often they employ professional questioning tactics to make it more likely you will lie — for instance, by phrasing the question or employing a tone of voice to make the fact sound sinister. You — having already been foolhardy enough to talk to them without a lawyer — obligingly lie about this fact. Then, even though there was never any question about the fact, even though your lie did not deter the federal government for a microsecond, they have you nailed for a false statement to a government agent in violation of 18 USC 1001.
If you don't talk to the government, there is no danger that you'll slip up and talk yourself into a felony conviction. Yes, the risk is less if you're a lawyer and know what to watch out for, but why risk it? There is literally no upside. After all, no federal agent worth her salt is going to change her investigation just because a potential suspect "cleared up some misunderstanding" with a quick phone call.
