Do you know what the hook is for on top of some post boxes

 Yes, the hook on top of some post boxes is called a "letter flag" or "flag indicator". It is used to signal to postal workers that there are letters ready to be collected. When a letter is posted through the slot, it raises the flag. This helps postal workers to quickly and efficiently identify post boxes that need to be emptied.

There are two main types of letter flags:

• Spring-loaded flag: This is the most common type of flag. It is attached to a spring, so it automatically pops up when a letter is posted.
• Gravity flag: This type of flag is not spring-loaded. It is simply attached to a hinge, so it relies on gravity to fall down when a letter is posted.

Letter flags are typically made of metal or plastic. They are usually painted red or orange, so that they can be easily seen from a distance.

In addition to signaling to postal workers that there are letters ready to be collected, letter flags can also be used to communicate other information. For example, some postal workers use the flags to indicate that they have collected the letters from a particular post box.

Letter flags are a simple but effective tool that helps to ensure that mail is delivered efficiently. They are a reminder of the important role that postal workers play in our communities.