Ekev | How Do You Spell "Mezuzot"?

Rashi, in his comment on Devarim 6:9, infers from the written form of "Mezuzot" - מזוזת - that we need only write the words of שמע...ואהבת (and the rest) on one doorpost, since the written version alludes to "Mezuzat". However, our Sifrei Torah all have that word written מזזות. We explore the phenomenon of כתיב מלא וחסר - (plene and "defective" orthography), where Hebrew letters are used to "guide" reading and are read as vowels. We also explore the rabbinic history of variant texts of the Torah and how that challenge was met.