Put all of the ingredients into a power blender (I use the Optimum 9400 Frooothie) and blend for 30-60 seconds on high, adding a little extra water if necessary to loosen until the mixture is smooth. Pour into a cup or glass and drink immediately.
Notes
If you don't have a power blender and want to use a normal hand blender, peel and grate the apple and peel the cucumber to help things along.