Here's why:

* Period of a Simple Pendulum: The period (the time it takes for one complete swing) of a simple pendulum is determined by its length (L) and the acceleration due to gravity (g):

```

T = 2π√(L/g)

```

* Mass Doesn't Appear: Notice that the mass of the pendulum (m) is not part of this equation. This means the period is independent of the pendulum's mass.

What changes when you double the mass?

* Potential Energy: Doubling the mass will double the potential energy stored in the pendulum at its highest point.

* Kinetic Energy: Doubling the mass will also double the kinetic energy at the lowest point of the swing.

* Momentum: The momentum of the pendulum will be doubled.

In summary: While doubling the mass affects the energy and momentum of the pendulum, it doesn't change how fast it swings (its period).