Exposure to light - they need to be stored in a dark, cool place. Don't eat any that are green, they're poisonous...

Hi, welcome to GoY, I agree with Bamboo, the green colour is chlorophyl, which isn't poisonous in itself, but it does show that the potato has produced solanin, which is, although you would have to eat quite a lot to suffer the nausea, vomiting, diarrhoea, and convulsions that can result, you probably wouldn't be able to, because the bitter taste would put you off eating too much, Derek.

thanks dave b