A type of food that squirrels eat. In the first film Scrat was trying to hide it for the winter. In the second film he was having a problem trying to protect it from peril. In the third film he fights over it with his crush Scratte. In the fourth film he was looking for Scratlantis, a place with loads of acorns.
The acorn seems to always be the main objective for Scrat in each film. But he always fails to retrieve it due to his act of clumsiness and stupidity. Scrat never seems to give up on retrieving it however. No matter how many times he receives a new injury from trying to retrieve it, he never even bothers to give up on it even though he should know that he will get hurt each time by now. He has never even grown tired of it once.