KING SCIENTIFICRubber Bung/Stopper, Pack of 10, 21MM x 24.5MM x 28MM, Waterproof, BS2775 Compliant

I am a science teacher and I bought these for my students to make bottle rockets. These are the perfect size to act to use stoppers in the opening of a regular water bottle. These ones are very good as they do not allow any of the gas to escape prematurely which means that you get maximum push when the bottle finally launches:)