why does the array elements change after block?

I am trying to solve this problem

Given a sentence containing multiple words, find the frequency of a given word in that sentence.

Construct a method named 'find_frequency' which accepts two arguments 'sentence' and 'word', both of which are String objects.

Example: The method, given 'Ruby is The best language in the World' and 'the', should return 2 (comparison should be case-insensitive).

Hint: You can use the method Array#count to count the frequency of any element in the given array.

Since the comparison should be case-insensitive. I use these code to help:

word = "the"
word_set = []
word.size.times do |i|
  word[i] = word[i].upcase
  word_set << word
  word[i] = word[i].downcase
end

Inside the block every time after upcase method the word does change and does add to the word_set, however when the block finish the word_set just contain the the the What is the problem?

via Chebli Mohamed