When magnesium burns in oxygen, magnesium oxide is formed. The magnesium oxide is heavier than the magnesium you started with. You can burn the magnesium in a crucible, and see this for yourself. One of the problems with this though, is that for the magnesium to burn, it needs oxygen, and this doesn't take long to get used up in the crucible. The lid needs to be removed periodically to let more oxygen in. Magnesium oxide is a solid, but it's quite powdery, and in opening the lid, you risk magnesium oxide escaping.
Such an occurrence happened to day. So I faked the results that I gave to the children.
It is true that I could have gone into why the end result was lighter, but there wasn't the time left in the lesson to do this, and the key point the children had to learn today was that chemical reactions can result in a change of mass.
They had seen how copper (II) sulphate pentahydrate crystals, when heated, lose water, and get lighter (and in so doing go from a lovely blue, to a white powder). The magnesium was a demo to show that things can get heavier, and thus, they will (hopefully) see that the mass can change (and ultimately realise that the mass of reactants and products is the same, but that the gas that escapes, or air that reacts, isn't getting way).
Now, I fudged the result on the spot. Had I been cleverer, I could have used the relative atomic mass to make up a plausible result. As it is, I didn't, so my deceit can be easily worked out. In retrospect, I should have been prepared for this, so that, even if I did have to fudge the result, a realistic figure could have been given. Currently it looks like there's twice as much oxygen there as there should be.
In future, I may well have to fake results again - this is because equipment isn't always accurate, errors can creep in, and I could just have poor technique on my part. If I am to make up results so the children can learn, then they should at least have a bearing on reality. In addition, doing so in this fashion doesn't seem like a lie to me - I am not trying to get children to form beliefs that are not true, quite the opposite.
I doubt any of the children in the class are thinking about the quantities of oxygen and magnesium involved, but if they are, they would be getting a misleading impression about how the universe actually works.
It's always best not to lie.