[Gil4]

“where is the scripture that tells us that Mary will crush the serpent?”

I hate to even lend a little credence to that bizarre interpretation, but my guess is it is VERY loosely based on this:

Genesis 3:14–15 (NIV) So the LORD God said to the serpent, “Because you have done this, “...I will put enmity between you and the woman, and between your offspring and hers; he will crush your head, and you will strike his heel.”

[Campion]

As my link explains, it isn’t really bizarre. The Hebrew pronoun is ambiguous as to gender, and the possible antecedents — grammatically, not theologically — are either feminine (”woman”) or neuter (”seed”), not masculine.

[the invisib1e hand]

it's actually the simplest, most literal interpretation -- about as "scriptural" as one can get.

but don't let reality upset your apple-cart of prejudice!