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Instant cool-down: Hot drinks and foods can be rapidly cooled down by setting them outside for a minute or two. For instance, Haley and I were having a "car picnic" the other day, and my Wendy's chili was too hot. I opened the car door, set it outside, and a few minutes later it was the perfect temperature. Works great for the kids' hot chocolates too.
Ice at the ready: If you ever run out of ice in the freezer, you can simply scrape some from the 1/4-inch sheets forming on the INSIDES of the windows. Usually only present at 10 or more below, though. (And no, I've never actually done that.)
No-aroma garbage: Who needs expensive, stench-blocking bags for soiled diapers? Ours freeze solid not long after being deposited in the garbage can in the garage, leaving little lingering scent.
Non-cheesey sippy cups: Admit it, you've all lost a sippy cup filled with whole milk in the car at some point, only to find it weeks later in a nasty stage of curdling. During the cold months here, ours just freeze and stay frozen when forgotten in the car. They roll around a lot, but aren't terribly unpleasant to clean out.
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Update 2/19 - Rob took this picture today. It was so beautiful I called him from the car and asked him to go outside from work and take a picture. I've never seen one so full before, almost a complete circle.