Decoding Calvin's secret code

Calvin often talks in a secret code with characters of his imagination. In this script, Calvin is talking in secret code to one of his imaginary characters 'Fanged Terror' to help him out in his class test. Check out the last block of the script, you can easily decode his secret code using a little imagination.

IXSAY - SIX
INUSMAY - MINUS
OURFAY - FOUR
URRYHAY - HURRY

Enjoy the script....