Mercury barometer

Okay, so you know how air around us has weight? It's like when you try to carry a big balloon around, it's heavy, right? Same thing with air. And when there's more air on top of us, it's heavier than when there's less air.

A mercury barometer is a tool we use to measure how heavy the air is. It's kind of like a tall straw with one end closed. We fill the straw with shiny, silver metal called mercury. When we turn the straw upside down, the heavy mercury slides down to the bottom of the straw, leaving some space on top for air to go in.

Now, the air from outside can push down on the mercury and fill the empty space at the top of the straw. The more air there is, the more it pushes down on the mercury. We use a ruler to measure how tall the mercury is inside the straw. If the mercury in the straw is taller, that means there is more air on top of it, which means the air is heavier. If the mercury is shorter, that means there's less air on top of it, which means the air is lighter.

People have been using mercury barometers for a long time to help them predict if the weather will change. When there's high air pressure, it usually means the weather will be clear and sunny, and when the air pressure is low, it can mean there will be storms and rain.