A few months ago, my husband and I went shopping for mattresses. Our old one dated back 15 years or so, and we knew it was time. One of the new kinds of mattress we wanted to check out was the type that gives you a different firmness "number" for each side of the bed. We liked the idea of being able to customize it, since I like a softer bed than my husband does. We liked the feel of them, but we ultimately decided to go with a more traditional mattress. The number ones work on air, which means they run on electricity. We didn’t feel it would be environmentally friendly to add another plug-in appliance to our home. We bought a memory foam mattress instead, and we love it.