I am new to ruby on rails and started a tutorial and found out the following function

```
 def string_shuffle(s)
>>   s.split('').?.?
>> end
=> nil
>> string_shuffle("foobar")

```

What does **`s.split('').?.?`** do? I know the split method but i have no idea what the two question marks are for. I saw "? used in boolean methods, but i couldnt understand how this one works. I tried to google it but, I couldnt find it at all.