A dry, indehiscent simple fruit consisting of one seed surrounded by a hard and thick pericarp (fruit wall). The seed does not adhere to the pericarp but is connected to it by the funiculus. A nut is similar to an achene but larger. Acorns, beechnuts, chestnuts, and hazelnuts are true nuts. Informally, other edible seeds or dry fruits enclosed in a hard or leathery shell are also called nuts, though they are not true nuts. For instance, an almond kernel is actually the seed of a drupe. Its familiar whitish shell is an endocarp found within the greenish fruit of the almond tree. Peanuts are actually individual seeds from a seed pod called a legume.

Did you mean hard nut to crack?