ReverseWords

public static string ReverseWords(this string sentence)
{
    var words = sentence.Split(' ');
    Array.Reverse(words);
    //string reversed = words.Aggregate((workingSentence, next) => next + " " + workingSentence);

    return string.Join(" ", words);
}

var sentence = "the quick brown fox jumps over the lazy dog";
sentence.ReverseWords();