Beginner Gardening: sunflower seedlings flopping over, 1 by pokokpenyu

pokokpenyu wrote:
my sunflowers (helianthus annuus) germinated about 5 days ago. they were growing fine until today, when they started flopping over. help!

what could be the problem? i don't think it's damping off because the soil doesn't seem too wet. the cotyledons look kinda heavy for the stems though... is this normal for the seedlings to flop over? should i try to stake them? should i try to repot them? should i add more soil? any advice appreciated!

a close-up shot of the seedling can be found here: http://pokok.gejala.org/2007/pokokpenyu/seedling/my-seedling...