资料题：When the sun is halfway across the sky, it is（　）。

Sometimes we can find our way with the sum.Perhaps birds and animals do the same.They don't have clocks like our clocks.But their bodies can tell them the time.These body-clocks can say to the animals,“There's half-a-day more of daylight.The sun is up there.So home is over there.”Their body-clocks are never wrong.Sometimes days are long and hot.Sometimes they're short and cold.But the body-clocks of animals and birds are always right.Sometimes it rains and we don't see the sun all day.Ir's behind gray clouds so we can't see it.Sunlight still comes through the clouds, but the clouds change the light.Sunlight through clouds is polarized light (偏振光).Men can't see polarized light, but some animals can see it.To our eyes, all light is the same.On a day of cloud, our eyes tell us, "It's daytime" .But where is the sun in the sky? We don't know.Some animals know.They look at the same sky and the polarized light tells them,“It's daytime, and the sun is halfway across the sky".These animals know the place of the sun behind the clouds.They can read is path across the sky.So they can find their way on days of cloud.