Streamlining
Streamlining is shaping an object so air or water flows around it with less resistance.
Think of it this way
Hold a flat plate under a running kitchen faucet and water splashes everywhere, pushing hard against your hand. Now hold the curved back of a soup spoon under the same stream. The water hugs the round shape and slides right off with barely any push. That smooth curve is why fish and fast cars are rounded, not flat.
Explaining streamlining by grade level
Think about a car with a smooth, rounded shape. Air slides right over it. A boxy shape pushes against the air and slows down. Smooth shapes move faster because air can flow past them.