Which of these four statements are true?
A. Water is a liquid.
B. Air is a gas.
C. Water is a fluid.
D. Air is a fluid.
Many people would say D is False. In fact, all four statements are True. Terms like gas, liquid, and solid are states of matter, whereas “fluid” describes how a material behaves. That is, a fluid can easily move, or “yields easily to external pressure.” Therefore, under many conditions we find that the equations for fluid motion are the same for liquids and gases, we just usually don’t consider compressibility when we do the calculations with liquid.
I bring up this idea of fluids not necessarily being the same as liquids because we need to talk about buoyancy, which we normally think of when something is floating on water, a liquid. However, the concept of buoyancy works with all fluids and is a central concept of physics that makes our balloon ride to space possible.
When a material is in a fluid environment it will move depending on the density of the material compared to the density of the fluid around it. Materials that are denser will descend, less dense materials will ascend, and when the two densities are equal then the object will be neutrally buoyant. This is obvious enough, but the implication can sometimes be hard to understand. The consequence is this: The buoyant force on an object is equal to the weight of the fluid that was displaced. That may sound familiar because that is Archimedes’ Principle. The net force on the object is the buoyant force minus the weight of the object that is doing the displacing.
For example, if a toy boat weighs 500,000 Newtons (N) then it will displace 500,000 N of water when it is floating. This might seem very different than a balloon, but if we look into it we will find that it is not. A balloon might displace 5 Newtons (N) of air but only weigh 3 N then it will have 2 N of lift. The way this is done is by displacing the air (mostly Nitrogen and Oxygen) with some other gas that is lighter than air. Helium is an extremely common and safe gas to do this, while Hydrogen is sometimes used in some large-scale or upgraded balloon operations. Hot air balloons become buoyant by heating regular air, which causes it to expand and reduces its density. Buoyancy is also the focus of a classic brain teaser: When you throw an anchor from a boat into a lake, does the lake level increase or decrease?
Let’s now consider the entire balloon and the payload connected to it. An astute observer might note that if the system starts off as buoyant it will ascend, causing the buoyant force to decrease because the density of the displaced fluid (air) is decreasing. The system will continue to ascent until the buoyant force is equal to the weight of the system and then it will be neutrally buoyant and stay at approximately the same altitude. In a system with a rigid balloon this would be true, and this type of system exists, but our balloon is made out of latex and can stretch from the size of a tiny passenger car to be the size of a modest house. Since helium and hydrogen will always be less dense than air kept at the same temperature and pressure, the balloon will remain buoyant as it stretches to allow the gas to take up more volume. Latex is an amazing material, but it cannot stretch forever so it will eventually pop high in the stratosphere (around 90,000-100,000 for most missions if everything goes to plan) and then your system will return to the surface of the Earth.
 Google definition result for “fluid”
 I feel like is a great time to tell you that fish do not float. Fish are neutrally buoyant. Dead fish float.
 Continuing this buoyancy math can also show that we feel about 0.1% lighter living in our atmosphere at sea level than we would if there were no atmosphere.
 When the anchor was in the boat it was displacing an amount of water that would weight the same as the anchor. When the anchor is thrown into the lake it is only displacing the volume of the anchor. Therefore, the lake level will decrease a very tiny amount when the anchor is thrown into the lake.