Answer
Here are the contents of the containers after each stage:
A = 9 litres, B = 4 litres
A B
9 0 fill A
5 4 fill B from A (leaving 5 litres in A)
5 0 empty B
1 4 fill B from A (leaving 1 litre in A)
1 0 empty B
0 1 fill B from A
9 1 fill A
6 4 fill B from A (only 3 more litres fit in B)
6 0 empty B
This leaves 6 litres in container A.
?
Puzzle 2
Starting with WARM, change one letter at a time, and end with COOL?
Each change leaves the other letters in their original places and must result in a proper word.