Flying is a fast way to travel.But it is a big headache for some people.They can't help worrying about things like "Is it dangerous if we don't change our mobile phones to airplane mode（飛行模式））？" Here are some interesting facts about flying.
    Why are plane windows round？
    It's for safety，as the airplane flies higher and higher，the atmospheric pressure（氣壓）outside gets lower and lower.During the flight，the pressure inside the plane becomes greater than the pressure outside.When this happens，square windows can break.That's because having different pressures inside and outside puts stress on the comers of square windows.But rounded corners can spread the pressure more evenly（均勻地）across the window's surface，making it less likely to break.
    What happens when a phone isn't in airplane mode？
    Some passengers believe mobile phones and laptops need to be turned off because they can bring the plane down.But this is not true.If you don't change your phone to airplane mode，pilots and air traffic controllers may hear unpleasant sounds in their headphones.This makes it harder for them to communicate.So it is still best to put your devices in airplane mode.

Facts about（1） flying flying
Round windows	◎Purpose：It's for（2） safety safety ◎Reasons：Round windows don't（3） break break easily in higher pressure.
Phones in airplane mode	◎Purpose：This makes it（4） harder harder for pilots and air traffic controllers to communicate. ◎Reasons：If people don't keep phones in airplane mode，both pilots and air traffic controllers will hear some（5） unpleasant unpleasant sounds.