Can you feel your heart beating? We mostly don't think about it. Like breathing, our heartbeat, pumping fresh blood around our body, is something that we live with, every moment, throughout our whole lives.
Some people think of their heart as their best friend. But hearts can get sick and then we are in real trouble. Maybe you heard about the footballer from Denmark who fell very ill because of a heart problem during a big match last week?
But there's also been good news for people with heart trouble. Doctors have just started to replace hearts with a new machine that keeps people alive. Imagine - a machine inside your chest that keeps your blood flowing, just like your old friend, your heart.
Jérémy's dramatic birthday
That's what happened to Jérémy Hage. It was the day of his 30th birthday and he was looking forward to spending Christmas with his little boy at their home in the north of France. Then, suddenly, he got very unwell. He had to be rushed to hospital. The doctors saw that Jeremy's heart was pretty much useless.
Now, since the last century, doctors have got better and better at repairing hearts. And when they're really poorly, you can even get a new one.
These are called transplants. That means that someone who dies, maybe in an accident, can donate their healthy heart. People who want to help others can be organ donors. That means that if something bad happens to them, at least they know that their heart, or maybe other organs, like their liver, can help someone else.
Good luck for Christmas
But in many countries, like France where Jérémy lives, there are more people who need new hearts than there are healthy hearts to go around. The future for Jérémy and his family last Christmas looked very worrying.
But Jérémy was lucky. The hospital where he was taken was trying out this amazing new device. It's called the Aeson. It's been designed over the past 13 years by Professor Carpentier and an engineering company from the group that makes Airbus jet airliners.
After an operation, Jérémy was soon able to get up and stroll about the hospital. He carried a little bag. This contained the battery that was wired to the mechanical heart inside him. Pump, pump, pump went the plastic and metal heart, just like the real thing - beating faster, for example, when he started to do exercise.
Machines for life
There have been other machines which can keep people like Jérémy alive for a while, but this new one brings them much closer to the freedom of having a real heart. In Jérémy's case, it allowed him to have Christmas with his little boy and then, last month, to finally get a transplant of a real heart from a donor.
Professor Carpentier hopes that one day mechanical hearts from his company, Carmat, will be so much like the real thing that people won't need donor hearts at all any more - they'll be able to live a whole life with the little machine pumping their blood.
Jérémy said he's thrilled to have been among the first to use this new solution, not just for himself but for his son: