Why does water bubble in the toilet while other drains are being used?

Water bubbles in the toilet when other drains are used due to changes in pressure within the Plumbing system. When a drain is used, it can create a vacuum or reduce pressure in the pipes, causing air to be forced into the toilet bowl, resulting in bubbling. This is often a sign of a partial blockage in the plumbing or venting issues, which can disrupt the normal flow of water and air. Proper venting ensures that air pressure remains balanced, preventing such disturbances.