DAYS = ['日', '月', '火', '水', '木', '金', '土'] DAYS << '日' p DAYS #=> ["日", "月", "火", "水", "木", "金", "土", "日"] DAYS = ['日', '月', '火', '水', '木', '金', '土'].freeze DAYS << '日' p DAYS #=> /Users/naoty/workspace/misc/freeze_sample.rb:3:in `<main>': can't modify frozen Array (RuntimeError)