You have a 4 liter bucket and a 5 liter bucket and an (infinite) supply of liquid.
How do you get 2 liters?
Fill the 4 liter, and dump it into the 5 liter. Then fill the 4 liter and pour from that to fill the 5 liter, leaving 3 liters in the four liter bucket. Empty the 5 liter, then pour the three liters from the 4 bucket into the 5 bucket. Fill the 4 liter again, and pour from that into the 5 bucket leaving 2 liters in the 4 bucket.
Not sure this question qualifies as a puzzle, but...
Fill up the 5L. Fill the 4L from the 5L. Empty the 4L. Now you have 1L in 5L and empty 4L. Now pour 1L from 5L into 4L. Then fill the 5L and then pour into 4L until it is full. It will hold 3L more, so 5L will now have 2L. Empty 4L. You now have 2L in 5L bucket. Success!