single PM DC motor?

I took apart a DC motor of questionable quality and I noticed that it uses only one permanent magnet. Basically a round cylinder covering the whole circumference of the steel can that it sits in. How can this work? I thought you'll need two permanents, one with the polarity N and another S to produce a consistent magnetic field. Can anybody tell me if these motors were designed badly or maybe somebody can educate me on how it can work. Thank you.