Palindrome Validator
You can call reverse on String directly.

While it doesn't particularly apply here, instead of splitting on '', you should use the method chars. 

Who says you'll be passing a sentence? Does this method not work for single words? If I call palindrome? 'radar' will I get an error?

I'd recommend using tr to strip out all non-alphabetic characters, instead of just deleteing spaces.

def palindrome?(testing)
  stripped = testing.downcase.tr('^a-z', '')
  stripped.reverse == stripped
end