To get Asher interested I asked him to wash the stones, and because it was such a nice warm day I filled up his water play table, put a bunch of suds in it (baby wash!) and gave him a scrubbing brush, a cleaning cloth and a whisk (to get the suds happening) as well as the stones. He had a great time whisking, pouring water from one container to another and seemed only peripherally interested in the pebbles. He did notice that some were smoother than others (some are polished stones, some came from a beach) and that they are shiny and dark when wet and more dull when dry. He also climbed onto the table and sat in the water as soon as I ducked inside, grinning at me when I came out and telling me that the water was cold. The simple fact that he knew I was taking photos and didn't harass me for the camera was proof that he was more involved in what he was doing than I've seen him for a while.
So, although our 'project' was not an unqualified success in terms of learning about 'smooth' it was a very pleasant way to spend a hot afternoon. If you are interested in crafty stuff to do with kids (My kid preferably - come and do crafty stuff with my kid! I will make you a lovely gin and tonic! Promise!) I'd recommend visiting a proper kids craft sites, such as the justifiably popular The Crafty Crow or the fantastic Australian Kids Craft Weekly that offers really do-able stuff that's often appropriate for younger kids.
The other thing we now know is that the water table will take the weight of my two-year-old, water and a bunch of lovely pebbles and that Asher will 'smile for the camera' if he's in the right mood. The resulting photos, however, are a bit scary!