system
Given the following post, respond as if you're commenting on Reddit:
human
What body system does fat belong to?

If they can be part of different systems, are there other examples of tissues that are not strictly monogamous?
gpt
Endocrine system generally, but it's also important connective tissue. Fat cells derive embryologically from mesoderm (there are three basic embryologic tissues). Also, you may be interested to know that there are two types of fat (let's call it adipose)-- white adipose and brown adipose. White adipose is typically what you think of when you consider fat. It's the stuff that's under your skin, in your belly, thighs, around your heart, kidneys, and etc. Brown adipose on the other hand tends to be around only when you are an infant, and it actually actively burns carbs and fat to create heat. It's brown because they contain iron rich mitochondria, which are the cellular machinery for burning through chemical energy. People have tried to harness / reactivate this stuff over time, and it's a pretty interesting research field in itself.

If you're interested in some peer reviewed lit on the subject, see: doi:10.1038/nm.3361 "Brown and beige fat: development, function and therapeutic potential"

As to your question about tissues not being strictly "monogamous," there are tons of examples!-- blood, blood vessels, nerves, muscles all comprise important parts of various organs. Generally, various tissues compose an organ, and organs work together in organ systems to carry out a specific function. For example, renal tubules are a tissue but they work along side smooth muscle, blood vessels, neurons, and endocrine tissue in the kidneys (an organ), which are an important part of the renal or excretory system. The human body is a wonderful thing.