You don't actually *have* to move the middle rider first on a triple beam balance! It's just a common practice that makes the process more efficient. Here's why:

* Starting with the largest increment: The middle rider on a triple beam balance typically represents the largest weight increments (usually 100 grams or 10 grams). By starting with this rider, you can quickly get close to the target weight without needing to make small adjustments with the other riders.

* Efficiency: If you start with the smallest increment rider (usually the front rider), you might find yourself making many small adjustments to get close to the target weight. This is tedious and time-consuming.

In essence, starting with the middle rider allows you to:

1. Quickly narrow down the weight range.

2. Minimize the number of adjustments needed.

However, you can certainly start with any rider you want. The important thing is to adjust each rider until the balance reaches equilibrium.