Instruct: Classify the given Amazon review into positive or negative sentiment.
Query: Great Product!

Whenever I go out of the country, I usually get some sort of stomach illness. However, this pen was pretty much a life-saver for me. I took a month-long trip to a 3rd-world country and used this pen every day to purify my water in order to brush my teeth and drink water. I did not get sick once. This product is one of the best modern-technology investments i have made, and I highly recommend it to anyone to use for overseas and backpacking/hiking trips. It helps save the water supply, and helps prevent the overconsumption and waste of plastic water bottles. In addition, when used with AA lithium batteries, this thing goes a really long way.