When it's ground.
注：ground可以表示“地面”，也是grind（磨碎、碾碎）的过去分词形式。
更多英语谜语
What's the largest room in the world?
Why is the comet like Micky Mouse?
When the boy fell into the water, what's the first thing he did?
Where were you when the power was cut off?