Toharot, Chapter Three, Mishnah Seven
1) A child who was found at the side of a graveyard with lilies in his hand, and the lilies grew only in a place of uncleanness, he is nevertheless clean, for I could say that another person gathered them and gave them to him.
2) So also a donkey that was in a graveyard, his harness remains clean.
Section one: Although it seems quite likely that the child picked the lilies from a place that was unclean, and therefore the child is unclean, since the rule is that if a person cannot be asked if he touched an impurity he is pure, even this child is still considered pure. We can see here just how lenient the sages could be in matters of doubtful impurity.
Section two: Since you can’t ask the donkey if he went over the graves (unless the donkey happens to be from Shrek), his harness or any other vessels that are on him, remains clean.