What Is Matza- Why Did Rashi Wait

One of the Kellolim (rules) in Rashi, is, he explains a word , the first time it appears

in the Torah or Gemoro.

 Shmos12- 39 “ Vayofu es habotzeik” – ugos matzohs . Rashi explains Botzeik

Shelo Hichmitz  Korui Matzoh (unleavened dough is called Matzoh)

Rashi waited until pasuk # 39 to explain what a matzah is.

The word matzah appears in Breishis 19 – 3 “umatzhos ofoh vayocheilu”.

It also appears in this same parsha three times before the pasuk of Ugos matzohs.

1)Shmos 12- 17” ushmartem es hamatzohs” --  2) 12 -18 “boerev tochlu matzohs” –

3) 12- 20 “ b’chol moshvoseichem tochlu matzhos”.

Why did Rashi not explain what a matzah is in Bereishis or at least the first time in this parsha?

from my father R. Z.Y. Friedman a"h

Why did Rashi not explain what a matzah is in Bereishis or at least the first time it appears in this parsha?

Rashi  in Yoma 75a explains the word  Ugos, “mashma kodem afiyah” before it is baked.

 It is not necessary for Rashi to explain what a baked matzah is. Every child knows that an unleavened baked bread is called a matzah.

In pasuk 12 – 39 it says Ugos Matzohs , Rashi is explaining that not only an unleavened baked bread is called Matzah , an unleavened  dough is also called  Matzah