An inclinometer is a tool that measures angles. When your rocket launches, you need to know the angle between the ground and your line of sight to the rocket. That's called the angle of elevation.
You're going to build one yourself using everyday materials. It's simple, it works, and it's the same basic idea engineers have used for hundreds of years!
When you look through the straw at your rocket, the string always hangs straight down (gravity pulls it). But the protractor is tilted with the straw — so the string appears to point to a number on the arc.
Here's the trick: because the protractor is upside down, the number the string points to is not your elevation angle. You have to do one small calculation:
Elevation = 90° − 35° = 55°
On launch day: aim the straw at the rocket at its highest point, pinch the string gently to hold it in place, then lower the straw and read the number. Subtract it from 90° — that's your elevation angle!