What is the purpose of a rubber stopper?

Laboratory rubber stopper - Wikipedia

Small but Mighty: The Function of Rubber Stoppers

Types of rubber stoppers and common uses

Stoppers Fisher Scientific

PROBLEM SOLVER SIZE #6 Drilled Rubber Stopper - Hobby Homebrew

Rubber stoppers (10 to this package). Used in these models: Coastal, Drinking Water & Wastewater Treatment, Waste Management (Landfill & Recycling)

Rubber Stoppers #3 / Plugs (10 Stoppers) (9-6196)

Medicinal Rubber Stopper, For Medical Use Rubber Stoppers

What is the purpose of a rubber stopper?

Recessed Self-Adhesive Rubber Stoppers - BS17 - Bumper Specialties

1-3/4 in. Rubber Drain Stopper in White (1 per Bag) - Danco

The Chemistry Lab. - ppt video online download