why didn't cassie let her tears out when mama questioned her

Cassie didn't let out her tears because she had made a pact with her brother Stacey not to tell her mother about the caravan. She knew her tears would make her mother suspicious.

"'No'm,' I mumbled, feeling very clumsy and close to tears. I knew that if I let the tears fall. Mama's suspicion that something was wrong would be confirmed for I never cried about such a silly thing as a fall; in fact, I seldom ever cried."