Why the girls did not get ill until the day after eating the sandwiches which had been in their school bags all day?

The girls likely did not get ill immediately after eating the Sandwiches because foodborne illnesses often require time for bacteria or pathogens to multiply and produce toxins. Symptoms typically appear several hours to a few days after consumption, depending on the type of contaminant. Additionally, factors such as the girls' individual immune responses and the specific conditions of the sandwiches, like temperature and moisture, can influence the timing of any illness.