Start with both containers empty. Fill the 3 liter container. Pour it into the 5 liter container. (Now the 3 liter container is empty and the 5 liter container has 3 liters, and has room for 2 more liters). Fill the 3 liter container. Use the full 3 liter container to just fill up the 5 liter container, which takes away 2 liters from the 3 liters in the 3 liter container, leaving exactly 1 liter in the 3 liter container. That's what you wanted! Edwin