This is really a very simple trick to perform if you have the right gimmick. The canteen is really double chambered with a small hole on the outer chamber. The canteen is upturned to allow liquid to flow out, a thumb is placed over the small hole to keep to much liquid from coming out. When the canteen is turned upright again, the thumb is removed to allow liquid from the outer chamber to flow into the inner chamber again. The thumb is placed back over the hole and the canteen turned over again to let more liquid flow out. This can be redone many times to make it seem that the canteen keeps refilling itself from nowhere.
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