Experiment to see if our lunch drinks contain acid

We did an experiment to see if any of our lunch drinks contained acid. Teacher put some strips of red cabbage in a bowl and added some very hot water. The water turned purple. Then teacher poured this purple cabbage water into numerous plastic cups. We then poured a few drops of our lunch drinks in the cabbage water. If the water turned red/pink we knew that it contained an acid. We know that acids are bad for our teeth. We discovered that orange juice, apple juice, lemon juice and Ribena all contain acid and that the best thing for us to drink is water and milk. The water and milk contained no acids.