I will make note for Ruby ;)

Perhaps just constructing a new string would be better?

Earlier katas stated whether your function was expected to be pure, or whether the author actually copied the arguments beforehand. So @Hybr1d, you probably want to add this to the description. although this only effects Ruby (AFAIK).

t isn't a copy, but actually references the very same object as dna. See http://patshaughnessy.net/2012/1/18/seeing-double-how-ruby-shares-string-values for another example.

Try not to manipulate given String because test ares testing the opposite then. Since you manipulted it :). Create new String and return that.

PS: If you still have trouble look online if not enoguh then just pass solution and you will see what you did wrong ;)

Oh... don't change the actual input create new string and add to it :). Then it works... like bkeas told you

Does your solution manipulate the argument? Your function needs to be pure, e.g. it may have no side-effects on the argument or the environment. As far as I can see, you change dna inplace, therefore invalidating the argument used for the verification.

To be honest I have no idea why...But test work for me just fine post solutin here I will mark it as a spoiler...

Tell me what is wrong, the test seems fine for me?