It could be entirely reasonable to guarantee something like that. It's a figure of speech as well, and doesn't imply that they have a way of proving it. And why

It could be entirely reasonable to guarantee something like that. It's a figure of speech as well, and doesn't imply that they have a way of proving it. And why are you confident they are a "he"? Anyway, I think the onus is on you to specify a way of settling the bet since you offered it.