If you've ever had the misfortune to be pulled over by the police then you might be wondering why the cops touched the back of your car while walking up to your window.
Even if you haven't, then you've probably watched enough reality TV about the cops to see it happen a few times.
While there could be any number of reasons to do it, a police officer has explained why they always do it once they've pulled someone over.
Officer Tony Messer said it was 'probably one of the top three questions I get' about his job and he explained the somewhat grim reasoning behind the action.
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"These fingerprints is just in case the worst of the worst happens when you approach the driver," he said of the protocol surrounding a 'traffic stop'.
"If a driver was to flee after committing a crime, this would link the vehicle and your fingerprints together to give the detectives another piece of evidence."
Leaving your fingerprints and DNA on a car would certainly help an investigation should the worst happen, but officer Messer also invited other cops to comment and see if they had their own particular reasons for it.
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One agreed with him that they did it to leave fingerprints but they also said they touched the back of a car 'to be sure the trunk is closed to avoid a possible ambush'.
Another officer agreed that their main reasoning for doing it was to make sure the trunk of the car was as closed as it appeared to be in order to stop anyone from bursting out.
They said the 'DC sniper case is a good example' of why they did this, and other cops said that they also touched the back of the car to check if someone was hiding, or in certain cases, being hidden.
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A third cop said they did it to check if there were any potential kidnapping victims hidden inside, as not every person shielded from view was a potential attacker.
In an interview with News 6, traffic safety expert Trooper Steve Montiero echoed the explanations of Officer Messer and other cops in the TikTok video's comments.
He noted: "When law enforcement officers conduct a traffic stop, there are plenty of procedures that need to be done, not only for the safety of the violator, but for the safety of that officer."
Montiero then reiterated that the patting of the trunk is not only to make sure it's 'properly secured' but also to leave fingerprints and tie the vehicle and officer together.
Joe Harker