With each mass shooting, the political discussion turns to background checks. The law can vary depending on several factors. One thing many Americans wonder is: how do these background checks work?

When you buy a gun from a federally licensed seller, you’ll almost always have to submit to a background check.

Once you pick out the gun you want to buy, you fill out paperwork. It’ll ask the usual: name, address and birthday. But the application also asks about criminal history, substance use and mental health. Lying on that application is a felony that can come with major fines and incarceration.

Once the application is filled out, the gun seller submits it to the National Instant Criminal Background Check System, or NICS. It scans three databases for information and usually gives an answer within minutes.

"Approved" means the seller can move ahead. "Canceled" or "denied" would stop a potential sale.

But the result could also be “delayed." That means the FBI has three days to further investigate before giving an answer.

If the seller doesn’t hear anything within three days, they are legally able to sell the applicant a gun. Some say that’s an issue with the system.

Gun sellers can also turn away sales if they feel uncomfortable about the way a potential buyer is acting or talking.

State laws can be different and, sometimes, trump federal law.

Critics say the whole system falls short. They point out a denied check — and sale — doesn’t stop the person from trying to buy a gun in other ways.