Water can often be an issue for me as well. Just last weekend I took my ten-year old on a two day trip into the local mountains. He weighs just 68 pounds, so I figure his maximum load should be around 17 pounds (one quarter of his weight). I weigh everything down to the gram and have him at 12 without water -- then realize I'm hiking into a canyon where, while I'm reasonably sure there will be water, I'm not absolutely certain.

So I weigh him down with three and a half liters of water, putting him over his weight limit, at least for the first couple of miles. He did fine (plenty of water in the creek, after all), but it brought home again the importance of considering everything. Think I'll see if I can shave a few more ounces from his load before our next trip.