With two buckets with a capacity of 8 and 9 liters, how can you collect 3 liters of water from the river?

First, you need to collect a 9 liter bucket and pour its capacity into an 8 liter bucket.
In this case, 1 liter will remain in 9 liter.
Then we collect the 9 liter bucket again and pour it into the 8 liter bucket.
Since there was already 1 liter of water there, we can only pour 7 liters into it.
In this case, 2 liters of water will remain in the 9 liter.

Pour out all the water from an 8 liter bucket and pour the capacity of a 9 liter bucket into it.
In this case, there will be 2 liters of water in an 8 liter bucket.
Then we collect the 9 liter bucket again and pour it into an eight liter bucket.
Since there are already 2 liters there, we can only fill it with 6 liters.
In this case, 3 liters of water will remain in the 9 liter bucket.