"Nipples come in two forms: they're either made from brass or aluminum. Brass is clearly the stronger one - although it doesn't necessarily make a stronger wheel - and aluminum the lighter one. The weight difference is about 20 grams per wheel, but this comes at a small price. Aluminum is a substance that, in contrast to brass, can react to certain (wet) weather conditions, possibly making them 'freeze' after a certain period of time, making future adjustments to spoke tensions impossible. This is not a huge threat, but so aren't the weight savings... So unless you definitely need the lightest wheel possible, just go for brass."