The famous magician Dynamo has opened up about how a condition called Crohn's can make his life difficult.
He says that medication he takes to help with his Crohn's has changed the way that he looks.
Crohn's is a long-term condition that affects the digestive system, but you would not be able to tell by looking at somebody that they have it. This is what is called an invisible condition.
Coeliac, sickle cell anaemia, lupus and colitis are just a few more examples of other invisible conditions.
George, Maisie, Amiyah, Ayshin, Chloe and Bridget - who all have invisible conditions - tell Newsround about the things people who don't understand what life with their condition is like say to them, and how that can be frustrating.