The several key pieces of evidence within the story are:
The crushed grass.
The boulder with blood.
The red sweater matched the one found in Jill's room.
These were useful to investigators because they helped them to reconstruct the crime scene, understand the motive and nab the killer.
The class evidence in this story is the pail atop the hill and the individual evidence was the red sweater.
<h3>What is Forensics?</h3>
This refers to the scientific tests which are carried out usually at a crime scene to get more information about the nature of the crime and the perpetrator(s).