I use them in my elementary science classes for when we are studying temperature (by making personal thermometers), as well as when studying density. They are sturdy, reliable, and cost-effective. Love them!
OK, but REALLY tight for the 5mm glass tube
The product description of the tube says that it fits all stoppers - that is true for this stopper, but you MUST lubricate the tube or it will break - I used Vaseline - really tight fit, but it did go in...
Works great for attaching rubber tubes. Useful for collecting gas