Fluid mechanics
Fluid mechanics is like a science of how liquids and gasses move around. For example, when we fill up a glass with water and move it around, the water inside the glass moves too. It's like if the glass and the water inside it are both connected and moving together. In fact, when we change the speed of the glass, the speed of the water inside the glass changes too. This is because both the glass and the water are moving through each other. Fluid mechanics describes how they both do that and how they connect together.
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