Because they're supposed to be skating on grass, aren't they?
n. the grass is greener (on the other side of the fence)：草是那边的绿；这山望着那山高
He was very happy with us but wanted to see if the grass was greener elsewhere.
People always think that the grass is greener on the other side of the fence.
n. to let the grass grow under your feet：坐失良机；浪费光阴
Not one to let the grass grow under his feet, Sanchez had a busy time in his first weekend as Northern Ireland manager.
n. to put sb/sth out to grass：使退役；使退休
At the age of 60 he was put out to grass.
The Prime Minister refused to be put out to grass. Asked if he would quit, he replied: `The answer is No.'
They accused him of being a grass.
Keep off the grass.
We were sitting on the grass.
I need to cut the grass.
the long grass around the tent
He was repeatedly attacked by other inmates, who accused him of grassing.
He grassed on the other gang members.
She grassed the whole area, and planted a border down one side.
The asphalt playgrounds have been grassed over or sown with flowers.
the age-old techniques of retting, scutching, spinning, weaving, and finally grassing (or bleaching) the linen
He'd never grass a mate.
n. get off the grass!：别胡扯！