At the third and final US presidential TV debate, Donald Trump and Hillary Clinton didn't shake hands.
It is extremely unusual for presidential candidates to show such a sign of unfriendliness towards each other, as we would expect leaders to show each other respect - even if they are rivals.
But why do we shake hands in the first place?
Where did the handshake come from?
Archaeological ruins show handshaking practices being used as long ago as in ancient Greek times, as early as the 5th Century BC.
Historians have found images on items like ancient pots showing people touching hands to make deals, for example.
The traditional greeting as we know it today is believed to have come from when people used to use swords for fighting.
People would carry them in a case, called a scabbard, on their left side.
This meant they could draw their sword with their right hand, if it was needed.
Shaking hands, which is traditionally done with your right hand, became a friendly greeting because it was proof that you came in peace and weren't holding a weapon.
It was also a sign of trust that you believed the other person wasn't going to take their sword out to fight you either!
Manners expert William Hanson explains: "A handshake showed you meant the other person no harm. It's important today as it's a sign of trust and friendship."
When are handshakes used?
It's not just in politics where we see people shaking hands with each other as a sign of respect.
Before sports matches, you will usually see players shaking hands with each other, as well as people like referees.
Business people will shake hands with each other before and after meetings, and to agree business deals.
Just as Donald Trump and Hillary Clinton have not shook hands on more than one occasion, other well-known people have done the same, such as sports stars. It is usually criticised as it is not considered proper sporting behaviour.
Are there alternatives to handshaking?
Despite the handshake being very common, not every country uses this as a traditional way to greet people.
As Mr Hanson says: "Almost all countries shake hands, although in Japan they bow, and in some other Asian countries, like Thailand, they do the Namaste."
The Namaste is when the person greeting will usually say the word "Namaste" to the other, with their hands pressed together, and do a slight bow.
Some countries in the Middle East do shake hands, but it might not be as firm as we would shake hands in the UK.
In China, it's polite to shake hands more lightly too and it might last for as long as 10 seconds.
Other countries, like France, might also kiss on the cheek to say hello or goodbye.