Given Data

1. Input: river, sun, mountain, tree.
2. Step I: sun, river, mountain, tree.
3. Step II: sun, tree, river, mountain.

Question

What rule is the machine using?

Answer

It places words in increasing order of length.

Solution

1. Sun has 3 letters and moves first.
2. Tree has 4 letters and moves second.
3. River has 5 letters and mountain has 8 letters, confirming increasing word length.