Ergonomics
Ergonomics is the study of how to design tools and spaces so they fit the way your body moves.
Think of it this way
A kitchen counter set at the wrong height turns simple tasks into a strain. If the counter is too low, you hunch your back to reach it. If it is too high, your arms tire fast. A counter built for your height lets your elbows rest at a natural angle, so your body moves with ease.
Explaining ergonomics by grade level
Think about typing on a keyboard. If it sits flat, your wrists bend in a way that can hurt. A split keyboard has two halves that angle out. Each half lines up with your arm so your hands rest in a shape that feels right.