Complex sentence: Some stoppers used in labs have holes in them to allow the insertion of glass or rubber tubing, often used when a reaction is taking place in the flask or test tube and the byproduct or result of the reaction is desired to be collected.
Split into simpler sentences: Some stoppers used in labs have holes in them to allow the insertion of glass or rubber tubing. This is often used when a reaction is taking place in the flask or test tube and the byproduct or result of the reaction is desired to be collected.