Do fish get thirsty? They don't because they don't need to. But they do need to intake water to survive.
We get thirsty because it's our body signaling us that we need to go drink water. Meanwhile fish are already surrounded by water, they just absorb it from their surroundings and also take some from the food they eat. 

Did newton just invent the law of gravity from top of his head and people just believed him? Simple answer, no. People didn't just believe him and neither did he came up with the law of gravity on top of his head in one day. It occurred to him one day, that why do things fall straight down instead of going elsewhere or floating in space. This observation led to him creating the law of gravity. His theory of gravity wasn't composed of his independent ideas either. Various people contributed to it and the theory was also influenced by the theories of many people from the past. 
He wasn't just suddenly struck by this idea and he started running around telling people about it. 

Is cereal soup? I think not. That is why they both have different names. When we think of soup we think of vegetables/meat, salt and spices and water. All of which is served hot. But cereal is milk and whatever you put in it. They both are completely different things.

Is Lavender a plant, scent or a colour? It's all three. Lavender colour is called so probably because of the colour of the flowers. And so is the scent.

Are eyebrows facial hairs? Well, I don't see the wrong in calling them that. They are hair on our face so they can be classified as facial hair. But because they have a specified name so we refer to them with that name for clarity.