A massive assortment of rubber O-Rings.
O-rings are similar to rubber washers. They are used to seal joints between 2 parts to prevent leakage of gas or liquid. This 407-piece set provides a huge assortment of sizes and thicknesses. Individual compartments help keep the contents organized.
A typical hardware store will sell a kit of O-Rings like this one OR they will sell individual O-Rings for a large profit.
The average cost of each O-Ring in this kit is only about 4¢ but if the store owner sells them individually, the retail price will usually be 5x (or more)!