A joke's a very serious thing.

There are things that are so serious that you can only joke about them.

Existentialists are monumentally and monotonously serious; they don't like to joke.

A true Englishman doesn't joke when he is talking about so serious a thing as a wager.

Sometimes When You Are Serious, People Thought You Were Joking, But Sometimes When You Are Joking, People Thought You Were Serious

Running for me is a sport. It's not a joke. It's serious.