You are going to mix up some concrete, and in order to get it just right you need exactly 6 litres. But you only have a 4-litre and a 9-litre bucket.
How could you measure exactly 6 litres of water using only those buckets? You have an unlimited supply of water.
First fill the 9-litre bucket.
Then pour 4 litres over to the 4-litre bucket (there are now 5 litres in the 9-litre bucket), and then pour out the water from the 4-litre bucket. Again pour 4 litres from the 9-litre bucket to the 4-litre bucket and empty it.
There will now be just 1 litre left in the 9-litre bucket. Now pour that remaining 1 litre to the 4-litre bucket but this time keep it there.
Fill the 9-litre bucket again and then pour water to fill the 4-litre bucket to the top (this only needs 3 more litres).
The 9-litre bucket will now contain exactly 6 litres.