Hydrodynamics
Hydrodynamics is the study of how water pushes against objects as they move through it.
When you stir a spoon through a bowl of water, the water pushes back against the spoon. The faster you move the spoon, the harder the water pushes. Water also piles up in front of the spoon and swirls around behind it. This push and flow is what hydrodynamics studies.
Explaining hydrodynamics by grade level
Push a pointy toy boat through a bathtub. Now push a flat block the same way. The pointy shape slides through water with less push-back. The flat shape has to shove more water out of the way, so it moves harder.
Projects that explore hydrodynamics
Water pushes against objects as they move through it, and shape determines how much resistance they meet. A streamlined shape like a cone meets less resistance on the way down than a cube of the same weight. When you drop four wooden shapes into a tall water tube, the cone sinks fastest because water flows around it more easily.
Water pushes back against a boat as it moves forward, and the bow shape controls how much. A flat bow plows into the water, while a pointed bow slices through it. A V-shaped bow with a curved bottom produces the least drag because it parts the water instead of fighting it.