13. (6 points) The product of 2013 eights is a very large number. The unit digit of this number is $\qquad$
【Solution】Solution: Starting from 2 eights, the unit digits of the product of multiple eights are $4,2,6,8$, these 4 numbers cycle;
$$
\begin{aligned}
& (2013-1) \div 4, \\
= & 2012 \div 4, \\
= & 503 ;
\end{aligned}
$$

There is no remainder, so the unit digit of the product of 2013 eights is the same as that of the product of 5 eights, which is 8. Therefore, the answer is: 8.