As explained by the light socket:

1900: “Touch this, you’ll feel a tingle!” “Haha, you did it!”

1930: “Don’t touch that, you’ll get shocked!”

1960: “Light sockets should have a warning label.”

1990: “Light sockets should have a protective cover so people can’t get accidentally shocked when a bulb isn’t there.”

Today: “It’s a design defect since it’s possible to shock yourself at all.”