Stack and Sorting

A group of students wish to make use of stacks so as to sort a given sequence of distinct single digit integers, i.e. {$0,1,2,3,4,5,6,7,8,9$}. They decided to make use of two stacks, say $S1$ and $S2$. At the start, they pushed all the numbers onto $S1$. Then each time they pop the topmost number from $S1$ and push it onto $S2$ using an algorithm. The algorithm they designed says, push the top