What are the keywords in the following sentence? a herd of sheep graze on some green grass
grass, herd, sheep

What are the keywords in the following sentence? the plane flies between layers of clouds
cloud, fly, layer

What are the keywords in the following sentence? The crowds is in the middle of the town square.
crowd, middle, square

What are the keywords in the following sentence? Two men grind their boards on a ledge in the snow
board, grind, snow