My mum uses our old toothbrushes for the taps and the dog's teeth. (one for each!) I came back from uni, having forgot my toothbrush, walked out of the bathroom whilst chatting to mum and trying to brush them at the same time. The look on her face, then the look on my face when she told me the truth. I was not a happy bunny. And yes, it was the one used on the dog.