Why is the ice smooth during ice hockey games?
The ice hockey smoothing machine was invented in 1940 and is used to keep the ice rink smooth periodically between periods in the game.
### Instruction: Why is the ice smooth during ice hockey games?
### Context: 
### Answer: The ice hockey smoothing machine was invented in 1940 and is used to keep the ice rink smooth periodically between periods in the game.</s>