Weighing Jelly Babies Puzzle

The Puzzle:

Weighing Jelly Babies
Real jelly babies have a mass of 10 grams, while imitation jelly babies have a mass of 9 grams.

Spike has 7 cartons of jelly babies, 4 of which contain real jelly babies, the others imitation.

Using a scale only once (and minimum disturbance of the boxes contents!), how can Spike determine which cartons contain real jelly babies?
Do you have the answer? Check against our solution!