Sub vs Sub!

# Hello World Program in Ruby
def output(out)
    puts out;
end

thing = "https://www.google.com"

output("Thing: #{thing}")
output("Sub!:" + thing.sub("https://", "https://").to_s)
output("Thing: #{thing}")
output("Sub!:" + thing.sub("https://", "https://").to_s)
output("Thing: #{thing}")
output("Sub!:" + thing.sub!("https://", "https://").to_s)
output("Thing: #{thing}")
output("Sub!:" + thing.sub!("https://", "https://").to_s)
output("Thing: #{thing}")