A glass percolator is used in a water bong to enhance the filtration and cooling of smoke.

Here's why:

1. Smoke Filtration: The percolator breaks the smoke into smaller bubbles, increasing its surface area. This allows for more effective filtering as the smoke passes through water, removing impurities and ash.

2. Cooling Effect: As the smoke is diffused into bubbles, it cools down as it passes through the water, resulting in a smoother, less harsh hit.

3. Smoother Inhalation: The combination of filtration and cooling makes the smoke less irritating on the throat and lungs, providing a more enjoyable smoking experience.

In short, a glass percolator helps provide cleaner, cooler, and smoother smoke.