STEM investigators in 4th & 5th class

The investigators were looking at different foods for visible and invisible fats. They found that if fat was present in the food, it would stain the brown paper. The investigators tested their theory at 10 & 30 minute intervals. They then applied their critical thinking skills to figure out as to why apples, carrots and lettuce left a stain as they do not contain fat. Do you know why?