I have read in several texts that in order to show that a given functor from the category of schemes (over, say an algebraically closed field $k$) to $\mathsf{Set}$ is representable, it suffices to check that it is representable in the subcategory of affine schemes. How is this the case? I have not been able to find a proof of this fact, and it may be rather trivial, but I do not see the connection.