Why can't I just use Google Cardboard?
That's an option. Google Cardboard ($15; vr.google.com/cardboard) is a cheap and simple cardboard headset designed for uncomplicated viewing of VR content. But unlike the headsets from Zeiss and Samsung, Cardboard doesn't have straps, so you need to hold it against your face. It also lacks the high-quality lenses that create a clear image across your entire field of vision. So while it's inexpensive, Google Cardboard is best suited for viewing 360-degree images and short videos, both of which are now available on YouTube and Facebook. For games or videos lasting more than a few minutes (or however long you can hold that headset to your face), you'll want to upgrade.