Halloween material separation

It's half term for us, and with Halloween approaching, I thought I'd set up a simple activity for my sons.  The idea was to use the different properties of materials in a mixture to separate them.  I mixed some salt (sand or any fine-grained substance would work), some plastic beads and some steel nuts (washers, or small magnetic coins would also work) in a little "cauldron".  With a bit of help from some Lego props, and the Meg and Mog book (by Helen Nicoll and Jan Pieńkowski), the story was that the hapless witch had mixed the ingredients in her cauldron incorrectly and her spell didn't work, so she needed to separate them and start again...

I put out a few different things that could be used for separating - spoons, tweezers, a sieve and a magnet.  My 5 year old decided after a quick poke to see what was in the mix that a sieve was the way to go, and he separated the salt pretty effectively, aside from a generous smattering all over the table!  

Then he decided to try out the magnet, finding it stuck to the metal sieve but things in the mix moved too.  After a bit of deliberation, he tipped the beads and nuts into a plastic tub and then used the magnet.  His little brother was pretty impressed with his ingenuity, but they wanted to know what the recipe was for the right spell...

This is where the science turned into a bit of maths and general amusement, and I made up some numbers of beads, nuts and spoonfuls of salt to add, and got them to stir it in the cauldron, with eyes closed, and make up a spell.  With a little sleight of hand, I swapped the Lego witch for another figure, much to the boys' amazement.  This then turned into a request for a spell to switch her back into a witch, some more separation, making a new mix - with lots of counting and fine motor use in picking up small objects for the little one - and another swap with their "spell".  To my surprise, both actually closed their eyes and didn't seem to realise how I'd changed the figure.  Eventually the big one kept an eye open and proudly declared that he'd worked out my trick!