Obviously you don't use it.    ;o)

Try Googling clay for car cleaning, specifically "clay bar" or "auto detail
clay."  Appears to be some sort of poly clay.  Stuff runs upwards of $20 for
a soap-sized bar I and saw today that Aaron Brother's Art Stores has poly
(modeling) clay for $1.50 for a bar with about the same amount as the
higher-priced auto wax brands.

Zaino Brothers (their online wax shop) sells it as does Mother's Gold,
MacGuire's and a whole bunch of others.  They even sell foam pads that
attach to Porter Cable random orbit polishers that holds a bit of clay in
the pad along with a lubricant.  Supposedly the idea is that it replaces
polishing compounds and cutting agents that remove clear coats whereas the
clay doesn't.

B~