Q: What can you catch but not throw?
A: A cold!
Explanation: Are you familiar with the expression, to catch a cold? when you catch a cold you get a cold (runny nose, sneezing, maybe coughing, …) from someone else. Cold sometimes means the opposite of hot but in this joke it refers to getting sick.
Normally when you catch something, like a ball or a frisbee, you can throw it back to someone. You can catch a cold but you cannot throw it… although you can pass a cold along to someone else. So, if you are sick, if you have a cold, please take care of yourself so you do not pass it on to someone else.
This joke is funny because it plays with the word catch (catch a ball) and the phrase catch a cold. Stay healthy!
Here is an explanation of how you can catch a cold: