I have a 4x Magnifying Glass. We took it outside and focused the sun's rays onto a piece of paper to make it burn. Very much like in this video below.
We then tried on a different spot and could not get it to burn. So we came back in and wrote some more xs and other things on the paper with the pencil. Everywhere that we wrote, with pencil, went up in smoke and had a hole in it within seconds. We know that lead has something to do with that but we are not sure what it is exactly. We don't understand why we couldn't get it to burn without the lead.
Perhaps it's because it's the middle of winter here? The weather is in the 20's and there is snow on the ground. Perhaps it's because we are using a 4x maginying glass? It might not really be strong enough to burn the paper? Perhaps these factors are making it difficult for the paper itself to burn. However it burns within seconds anywhere there is lead. Perhaps the leads burning point is lower?? Or the lead is acting as an accelerant? Don't really know for sure, but he had fun burning the paper.
Later this week we will actually have a discussion on this and will be able to say remember Sunday on how we did that. If it's a sunny day when I plan on having the actual discussion we will go out and burn more paper. I just wanted to do it today in case it end up being a cloudy day later this week.