Can I ask questions associated with caffeine, like what are different sources of caffeine?
Or that kind of questions are off-topic?
An adjunct to @fredley's answer: You can ask questions associated with coffee.
The tour sums it in the first sentence:
Coffee Stack Exchange is a question and answer site for people interested in all aspects of producing and consuming coffee.
That's it. If your question doesn't involve producing or consuming coffee, it's questionable. Though coffee (does, generally) happen to contain caffeine, caffeine (itself) is not the topic of the site. Some other examples: Cola is off-topic, even though the beverage has a similar color. Tea is off-topic (see also original discussion), even though it has other similarities. Questions that discuss coffee, and happen to mention other topics (e.g., tea or cola) are fine -- if they discuss coffee as their primary topic.
That said, we do need to put some content at the help/on-topic page. This is the canonical location for detailed information about what is on-topic at a given site; it's shortcut-linkable in comments from
It depends really. Caffeine is many things. It is a chemical. It is a psychoactive drug. It is a taste. It is a component of coffee, tea and other drinks.
If you've got a question about caffeine, have a think about what context you're asking it in, and who might be able to answer. Would a chemist be the type of person who might know the answer? Take it over to Chemistry.SE.
If it's a direct health question (related to caffeine in general and not specifically coffee - which contains other chemicals that have other effects), this might not be the right place.
Your example "What are different sources of caffeine?" is a bad question for SE in general, as it's kinda open ended and the answer would just be a big list. Even questions like "What plants produce caffeine?" might do better on Gardening.SE.
If your question is about caffeine in relation to coffee in any way, it's probably on topic though.
It's still early days, we've got a long way to go in terms of finding and testing the limits of our scope. If you're not sure, post anyway. It's much better to discuss actual questions than hash imaginary ones out on meta :-)