I have some small beet roots that began to sprout leaves in the dark cupboard. If I plant them, will they grow into big beets?
They will more likely put up and inflorescence and produce seed. The seed may then be panted next year to produce eatable beets if you have the right species. Some species are bred for beet greens and never produce any eatable root of significance.
I find this sort of thing amusing as it doesn't take much space in the garden.