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Q: Why are airplane propellers in the front instead of the back?
A: A plane has an engine that powers the propeller. The engine creates power by burning fuel, which creates hot air. If the propeller was in the back of the plane, the hot air would be pushed away, so the plane wouldn't be able to move forward as quickly. Instead, having the propeller up front allows the propeller to push the plane forward. The pushing force of the propeller is called thrust and this is what moves the plane through the air. So the propeller needs to be in the front, because it creates the thrust needed for the plane to fly.
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Q: How can the outside of a record on a turntable move faster than the inside?
A: Imagine your record is like a pizza with cheese, tomatoes and onions on top. When you turn the turntable, it's like you're spinning the pizza around and around. The cheese and tomatoes stick to the top of the pizza and don't move as fast. The onions, however, are very small and light and they spin much faster around the outside of the pizza. It's the same with a record on a turntable. The outer edges are lighter and move faster, while the inner grooves (with the music) are much heavier and don't move as fast.
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Q: Why doesn't the ocean keep getting saltier over time?
A: The ocean has a really clever system that helps balance it so that it doesn't get too salty. In the ocean, a lot of water evaporates and turns into clouds, then it rains back down into the ocean again. The evaporating water takes some of the salt in the ocean with it, which helps keep the ocean from getting too salty. Also, when the water from rivers flows into the ocean, it takes some fresh water and dilutes the salt which also helps to balance the saltiness. In other words, when the same amount of salty water is going into the ocean and the same amount of salty water is coming out, the ocean doesn't get too salty over time.