What Can Be Seen, But Cannot Be Touched, And When Put In A Barrel, Will Make The Barrel Lighter?

Riddle: What can be seen, but cannot be touched, and when put in a barrel, will make the barrel lighter?

Answer: A beam of light!

Source: https://www.riddles.com/index.php/601

What can be seen, but cannot be touched, and when put in a barrel, will make the barrel lighter? — What can be seen, but cannot be touched, and when put in a barrel, will make the barrel lighter? by Anonymous v1.

Riddle: What can be seen, but cannot be touched, and when put in a barrel, will make the barrel lighter? Answer: A beam of light! — by Anonymous v2.