What did Lennie have in his pocket?

In chapter 1 of Of Mice and Men, Lennie carries a dead mouse in his pocket. When questioned by George, Lennie claims that he found the mouse (already dead) at the side of the road. He also says that he didn't do anything bad to the mouse, he just wanted to stroke it.
Lennie's fondness for stroking soft things has caused trouble for the pair in the past. They were run out of their last town, for instance, because Lennie stroked a girl's dress and frightened her.
The dead mouse, therefore, symbolizes Lennie's strength. Even though he does not intend to hurt any living creature, he does not understand the extent of his own strength. This idea will play a significant role later in the novel when he sits down and talks with Curley's wife in the barn.