Saw a video on youtube where some guy made a water filter out of a bottle, charcoal, sand and little rocks. He commented that since the charcoal is very porous it's great for filtering. Then, after constructing the filter he continued to filter some dirty water and commented that the first little bit is going to be nasty since its getting the dirt off the sand and rocks in the filter.
This doesn't make sense to me, if the charcoal is so great, and all that the rocks and sand bring to the party is dirt, why not just fill the bottle with all charcoal? Not to mention that charcoal is much easier to come by than various grains of sand in a forest or a jungle or whatever.