Monastery of Spinjitzu or in the Mountains of Impossible Height. They appear similar to real-world acorns, but with a long, crooked stem resembling the fuse of a bomb. They are implied to be extremely powerful explosives, and can be used as "Shurike-nuts" by experienced Ninja Squirrels.
All the power of a mighty oak tree, trapped inside the little shell of an explosive nut. In the paws of a Master Ninja Squirrel, Ninja Acorns are extremely dangerous; they become flying Shurike-nuts.