"thoughtful not joking" is not a sentence and it does not contain the word "serious". Serious is an adjective, a word that describes a noun. Example: Justin has a serious expression on his face. The adjective "serious" describes the noun "expression".
There are many people who are serious about school, while others are not.
"this play i serious so act like you care"
Serious answer: Wait as the car draws near. Non-serious answer: This sentence will end with the word 'near'!
He's so jocular that you can't get any serious sentence from him.
I'm in serious pain right now.
He was given a serious warning about his behavior.
A ruptured pulmonary artery is a serious problem
Habitual lying is a serious weakness in a person's character.
Tardiness is a serious and persistant problem at our school.
people with serious conditions should see a medical surgeon
Insubordination is a serious fault in the Army, unless it was justified.