What kind of weed is this

I just found it beside the driveway. It seems to have oddly-shaped leaves. There's one little flower at the end. I think it's yellow.

Here's two pictures of it:

http://tinypic.com/r/23wr435/6

http://tinypic.com/r/35bcf7o/6

Based on the appearance of the flower and the leaves, it's likely a wild lettuce (Lactuca sp.). These can have either yellow or white flowers.

Some of the lettuces that occur in Ontario that your plant resembles are

tall blue lettuce http://plants.usda.gov/java/profile?symb… (has pale lavender or white flowers)
Canada lettuce http://plants.usda.gov/java/profile?symb…
hairy lettuce http://plants.usda.gov/java/profile?symb…
prickly lettuce http://plants.usda.gov/java/profile?symb…

Some other lettuces that have leaves which don't resemble those on your plant
willowleaf lettuce http://plants.usda.gov/java/profile?symb…
woodland lettuce http://plants.usda.gov/java/profile?symb…
and, of course, garden lettuce which has a variety of leaf shapes.

I wouldn't recommend eating this plant, though, because of the age of the plant. Like garden varieties which have flowered, the sap can get milky and give it a very bitter flavor. Otherwise, you can eat these like the garden variety (just be careful to remove any spines if it's prickly lettuce).

A smokable kind.