Containers with matching lids are purchased.
The lids are removed.
Lids and containers are stacked for easy storage.
Stacked items are placed in the Tupperware cabinet.
The door is shut.
In the absence of light, the lids and containers begin to migrate away from one another. Within a few days, the original lids and containers mate, produce an offspring, and die. Upon death, Tupperware containers disintegrate into a powder which resembles ground graham cracker crumbs.
The offspring is always a different size than its parents; ie, two six-inch circular lids can mate and produce a 4-inch circular lid. Sometimes the offspring is a different shape entirely, so two square lids can produce a rectangular lid. In an interesting twist, however, the most common combination of lid- and container-offspring results in similar shapes but ever so slightly different sizes.
If enough time is allowed to elapse, the Tupperware cabinet will eventually contain hundreds of containers and lids no one can remember purchasing, and none of which fit together.
*With apologies to Douglas Adams