Suppose you have ten slow matches (also called match cord) and a lighter. If a slow match is lighted, it will burn for exactly one hour. The slow matches do not burn at constant speed, in other words it can happen that the first half of the slow match is burned in 15 minutes, while the remaining part will take three quarters. How can you measure exactly 45 minutes using these slow matches, and what is the minimum of slow matches you need for this?

It happens once in a minute, twice in a week, and once in a year?

What is it?

Who makes it, has no need of it.
Who buys it, has no use for it.
Who uses it can neither see nor feel it.