So if you were to donate 70 of your liver, it would regrow in about a year and on the flip side, if you got even 30 of a liver from a donor, those cells would create a full-sized organ in about a year. Why? Well, when you cut off part of the liver, the cells get a signal to start growing and multiplying until there's just a whole organ there again; you can't donate like infinite livers, though when the liver grows back, the shape and structure of the tissue aren't quite the same as in a brand-new organ. That's because when you first grow a liver, you use stem cells which got a cute, to make a very specific looking organ. But when someone regrows their liver, they're using mature liver cells, which are just multiplying to fill in a gap, so a regrow liver totally works. But if you tried to donate it, it probably wouldn't cooperate with the original liver tissue.