Our skin renews itself almost completely every 28 days.
Why it is true
Dead cells from the upper layer of skin (epidermis) are replaced by new cells generated in the lower layers.
Source: Dermatology
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.