1. The square of any natural number will end in 0, 1, 4, 6, or 9. It cannot end in 2, 3, 7, or 8. 2. If a number ends in 0, its square will end in an even number of zeros. If it ends in an odd number of zeros, it cannot be a perfect square. 3. If a number ends in 5, its square will also end in 5.