Milo thought that

We would have to test all 25 numbers to be sure what his rule was.  Do you agree?

Helen of Lea Valley Primary school had a good idea:

For the "I like these numbers" I put $\{2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24\}$ because they are even and for "I don't like" I put $\{1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25\}$ because they are odd.

Kate of Bilton Junior School exclaimed:

It is the five times table, well it has to be! What do you think?

Rischabh of  the European School of Varese was unsure and wants more information:

It is either odd numbers, or the five times table. I would like to try number $7$. If it goes on the left, it means it's the odd numbers, and if it goes on the right, it's the five times table. I will also try $10$ to be sure.

What if the number $7$ and the number $10$ went into the "I don't like" section. Are there any other possibilities?

Simran said:  The rule might be to select the numbers with units of $5$.

On the other hand what might the rule be if we tried $7$ and $10$ and both numbers went into the "I like" section?