what's the difference between thinking and thought?
p.s they're nouns
The word thought refers to a single thought and in this way it's similar to the noun idea. Thinking refers to the process of thinking. You can think of thoughts as products of thinking.

BTW, thinking is not always a noun. Sometimes it's a present participle

Thinking about how to spend the rest of the day, he walked out of the hotel.
"Thought" may be uncountable as well as countable.
I just had an interesting thought. (countable)
She was deep in thought. (uncountable)

Uncountable "thought" and the gerund "thinking" can be synonyms in some situations.
Thought/thinking is good for the brain.

"Thinking" as a present participle can serve as an adjective, or as part of a verb tense:
The thinking man prefers to make his own decisions. (adjective)
I have been thinking about going back to school. (verb)
Students: We have free audio pronunciation exercises.
Thought is a product of thinking , may be a sustained process , and again it reaches another thought . So the process may go on.
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What about "thought leader"?

Do you suppose that "thought leader" is a compound noun, whereas "thinking cap" is an adjective + noun?