Q: Why can’t you take anything balloons say seriously?
A: They’re always full of hot air!

Explanation: “To take something seriously” is to believe what someone is saying.  “To be full of hot air” is to talk a lot without saying anything of importance or to say things that are not true.  Balloons that rise into the air are literally full of hot air (or helium).  This joke plays on the phrase “full of hot air.”