Scientist: In our study, chemical R did not cause cancer in laboratory rats  But we cannot conclude from this that chemical R is safe for humans
After all, many substances known to be carcinogenic to humans cause no cancer in rats; this is probably because some carcinogens cause cancer only via long-term exposure and rats are short lived
It illustrates the claim that rats are too short lived to be suitable as test subjects for the carcinogenic properties of substances to which humans are chronically exposed.