Flags are used to symbolize many different things. Each of the elements in the flag represents something special. For example, each of the stars in the flag of the United States represents one of the States. The way a flag is displayed can also symbolize something special. A flag flown upside-down is a sign of great distress, and a flag flown at half-mast is a sign of mourning.
Mourning is when we show how sad we are for the death of someone important to us. Flags are normally flown at full-mast. This means that they fly right at the top of the flagstaff. Sometimes, they are flown at half-mast. They are flown at half-mast as a sign of mourning. That is a perfectly good answer to why we fly them at half-mast, but it doesn't explain the reasons behind it.
When we talk about flying a flag at half-mast people think of flying them half way down the flagstaff. This is how we do it today. In some countries they still follow the old naval traditions from way back in the time of wooden sailing ships. That said that a flag flown at half-mast should be lowered until it is one flag depth below the top of the pole, mast or staff. The reason is that there is room to fly one flag above it. This is a symbol, the flag that should be above it is the "invisible" flag to represent that the person is 'invisible" or no longer with us. Now in the United States there is a set of rules for exactly how to fly a flag, approved by Congress in 1942. They say that a flag at half-mast should be half way between the top and bottom of the flagstaff.
Another important thing to remember is that the flag represents our country. Our country is more important than any one person. So during the flag ceremony, when the flag is raised, it should be raised ALL the way to the top of the flagstaff, and then lowered to show respect for the dead person. When it is time to remove the flag, it should first be raised to the top of the flagstaff and then lowered and removed. This is to show respect for our country. Our country is more important than any one person or group of people.
The flag is a great symbol of national pride. It should be treated with respect. It should be flown correctly, and when raised and lowered, should not touch the ground. It should only be flown during daylight hours, and should not be flown during weather conditions that may tear or damage it. In special circumstances, and in order to show patriotism, the flag may also be flown at night, provided it is properly illuminated (lit up).
Flags can only be flown at half-mast to represent the death of an important government official. In order to fly it half-mast for any other reason, the President must give his approval. This week the President gave his approval for all flags to be flown at half-mast. It is being flown at half-mast to mourn the deaths of the people who died in New York, Washington, and in the airplane in Pennsylvania. Other countries around the world have also ordered their flags flown at half-mast this week. They want to show the American people that they care about what happened, too.