How can materials as simple as agarose gel, an electric field, and a DNA stain be used to separate and compare different fragments of DNA?

The negatively charged DNA fragments move away from a negative electrode and move towards the oppositely charged positive electrode in an electric field through the pores of the supporting material of the agarose gel. The movement of the DNA fragments is determined by their size. The DNA stain inserts itself in the DNA fragments and so helps in locating them on the gel. The distances travelled by separated DNA fragments are compared with the distances travelled by known DNA fragments to determine their size.