The acid in most fruit juices, especially citrus juices, will break down creatine into its waste product creatinine (note the spelling) before your body can absorb and use it. There is one exception (kind of)--grape juice. It has some acid to it and may break down some of the creatine depending on if you let it sit for a long time, but the sugar in grape juice is one of the most absorbable sugars in nature. It seems to help drive the creatine in--IF you don't let it sit. Other than that, it's usually best to just use water.