Reference water levels

Okay, imagine you have a big bucket of water. Now let's say you want to measure how much water is in the bucket. But, you can't just look at it and say "oh, that's about half full." You need a way to measure it more accurately.

So, what you do is you take a ruler or something else that can measure how tall the water is in the bucket. You see that the water is touching the 5 inch mark on the ruler. You write down "5 inches" as the water level.

Now, let's say you pour out some of the water from the bucket. You want to measure it again to see how much water you have left. But, you don't want to start over and measure from the bottom of the bucket.

So instead, you use the reference water level from before. You look for the 5 inch mark on the ruler (where the water was before) and measure from there. Let's say you see that the water is now at the 2 inch mark on the ruler. You write down "2 inches" as the new water level.

This reference water level helps you keep track of how much water you have in the bucket even if the amount changes over time.