What kind of weed?

Ok this is the first time that I have had the chance to do some landscaping. We rototilled and seeded the front yard a few weeks ago and now have some nice grass growing....and some weeds. This kind of weed wasn't there before, so I am almost thinking that they came with the grass seeds ...they are fat and juicy leaves and stems and if I don't keep up with them they will get yellow flowers. I can't used the weed kill stuff till I've mowed the lawn 3 times (one more to go) but I would like to know what kind of weed it is....anyone?

That looks like(actualy I'm pretty sure) buttercup.and ypu don't want it :no

Thanks for the heads up! Are there different kinds of buttercup? the flower on this appears directly on the crown of the leaves - buttercups I remember (only remember the flower, not the leaves) sends up an extra shoot, no? Wouldn't be the first time I'm wrong...

Are there different kinds?Some might call that a kind of understatement

Partial species list

* Ranunculus acraeus - a newly described species from Otago, New Zealand
* Ranunculus acris - Meadow buttercup
* Ranunculus alpestris
* Ranunculus aquatilis - Common water crowfoot
* Ranunculus arvensis - Corn buttercup
* Ranunculus asiaticus - Persian buttercup
* Ranunculus auricomus - Goldilocks buttercup
* Ranunculus bulbosus - Bulbous buttercup
* Ranunculus californicus - California buttercup
* Ranunculus cymbalaria - Marsh buttercup
* Ranunculus ficaria - Lesser celandine
* Ranunculus flammula - Lesser spearwort
* Ranunculus fluitans - River water crowfoot
* Ranunculus glaberrimus - Sagebrush buttercup
* Ranunculus glacialis - Glacier buttercup
* Ranunculus hispidus - Bristly buttercup
* Ranunculus lapponicus - Lapland buttercup
* Ranunculus lingua - Greater spearwort
* Ranunculus lyallii - Mount Cook Lily - reputedly the largest buttercup
* Ranunculus occidentalis - Western buttercup
* Ranunculus peltatus - Pond water crowfoot
* Ranunculus platanifolius - Large white buttercup
* Ranunculus pygmaeus - Pygmy buttercup
* Ranunculus repens - Creeping buttercup
* Ranunculus sardous - Sardinian buttercup
* Ranunculus sceleratus - Celery-leaved buttercup
* Ranunculus septentrionalis - Swamp buttercup
* Ranunculus sieboldii

To name just a few.
Some look real nice,but weed is as weed goes(all over the lawn)