why does everyone always remember that it is Sara who laughed?

i've always wondered why it is that everyone remembers that Sara (and not Abraham) laughed when she heard that she would bear a child. and even more so, why it is often seen as bad that she laughed. at least, that's what i expect a lot of people think - perhaps i have it wrong. it is what i often thought/remembered until awhile ago.

in Gen 18, Abraham gets some special visitors (Melchizedek and company) - and they tell them that within a year Sara, his wife, will have a son. Sara overheard them and laughed. when they asked her if she laughed, she lied about it (perhaps the lying was the bad part of the story - but i can imagine that lying was a lot more polite and hospitable to her guest than admitting that she laughed, which implied that she doubted not only the validity of the message and its source but might have even wondered if her guest was 'right in the head.')

prior to this story Abraham and Sara had tried to help God along with the promise of descendents - and Abraham bore a child by Hagar, Sara's maidservant (and those who know the story know that this was a bit of a disaster). but God promised again and again that Abraham would have children. and in Gen 17, the chapter before the one in which Sarah laughts, Abraham is told that Sarah would bear him a son. and Abraham laughs. and says, long live Ishmael (i.e. the son I already have). and God corrects him and says, no, Sarah will have a son, within a year. and the story ends with a list of blessings and promises and then Abraham goes home. the next thing we read in the story is the visitors to Abraham, the promise again of a son to Sarah and Abraham, and Sarah's laughter.

and it makes me wonder. did Abraham not tell Sarah about his talk with God and the promise? did Abraham not believe it himself (and did he believe it after it'd been confirmed by the messengers)? and why was Abraham not ashamed after he laughed but Sarah was? does it make a difference that Abraham already had one son and Sarah had none? how much pain had Sarah's childlessness already cost her? and did her laughter cover up her inability to dare hope anymore - she'd already spent years hoping and had nothing except Hagar and that son?

and it makes me wonder. perhaps some day i'll dig up some commentaries to see what they have to say, but for now it's enough just to look closely at the story and wonder. and i wonder what we miss when we only remember part of the story - or forget to see enough of the story that we stop wondering about the laughter - and why it is they named their child, Isaac, after laughter.