Our latest design incorporates over 20 improvements that have made Posi-Click tubes our #1 seller. Denville's new Posi-Click tubes seal easily with a distinct "click" and stay closed through incubation, freezing and boiling procedures. The unique closure system provides a tight seal that is actually strengthened by internal pressure. It will withstand boiling for 5 to 10 minutes. These tubes represent our fourth generations of improvements, incorporating over a dozen advanced features. Shouldn't you be using the best. Unsurpassed clarity. Frosted writing surface and graduations. Ultra smooth interior-low liquid retention. Made from highest quality, virgin, single melt polypropylene.