How about an easy one for a Monday! Common and important, unless you want to itch. Poison oak is easy/hard to identify, depending on how experienced you are in looking at plants. The leaves can range in color from dark green to light green to reddish, but they always have a glossy shine and another quality as well, one that’s distinct but hard to define. The leaves look… slightly luminous. Tender. As if, were you crazy enough to nibble on one, it would have a pleasant delicate texture.
The flowers of poison oak are small, inconspicuous, and not usually in bloom, so the leaves are what to look out for. These always come in groups of three, and they have a smooth surface, the veins only showing lightly and the surface not haired or spined. The edges are gently lobed or scalloped – to varying degrees, but they are neither straight nor serrated (like the edge of a bread knife). This helps you rule out other common plants, like blackberry, that some people confuse for poison oak. Once you get all those features down, really the only thing that looks similar are the leaves of a young true oak tree before it has grown tall.
The Latin name is easy to remember as well (even though I had forgotten it). Toxicodendron diversilobum. Intuitive, right? Toxic leaf with diverse lobes. Thanks John Torrey, Asa Gray, and Edward Greene who did the naming. Torrey produced the Flora of North America in the 1800s, and Gray helped him out. And how do I know they named our toxic friend, a newbie may ask? It says so right on all the official listings of the species. Check the names after the italicized species name on the above link: you’ll see Torr. and A. Gray, who did the original naming, along with Greene, who came along later and made some change – I’m not sure but maybe he is responsible for the “Toxicodendron”, since I seem to remember poison oak was a “Rhus” when I first learned it. Anyway this convention is called an author citation, with the namers being the “taxon authors“.
Flowers Of Marin 
My Berkeley friend first learned to be cautious in my company on our long hikes in the Berkeley Hills. He was extraordinarily sensitive to poison oak. I was not. He could spot it anywhere, and he taught me to as well, if we were going to continue our wild hillside explorations. I think he found poison oak, where I said not to worry, on our way out of that fog storm and into the sheltering redwood canopy. Damned if he was going to be fooled again when I abandoned our shelter spoiled by fog drip for the uncertainties of poison oak patches in the dark, cold, windy field. At least, not until he simply got rained out.