Avraham was Circumcised by a Scorpion

When Hashem told Avraham about circumcision, how did Avraham know how to do it? The Midrash Tanchumah on Parashas Lech Lecha, Sif 17 gives an amazing answer:

Avraham was sitting and wondering how he should circumcise himself. Once the Holy One, Blessed is He, told him, "I will place My covenant between Me and you," what is written following this? "Avraham fell on his face." When he fell on his face, the Holy One, Blessed is He, indicated to him `the place' and a scorpion bit him [there], so that he found himself circumcised (nimtza mahol). How do we know? From what is written, "God spoke to him saying, `As for Me, this is My covenant with you.'" Look, you are circumcised. And it is written, "On that very day Avraham became circumcised (nimol)." It does not say that he circumcised himself (mol), rather "he became circumcised" (nimol). ...

Thus according to this Midrash, Avraham did not circumcise himself, but rather Hashem caused a scorpion to do it for him. Ouch...