Just what is a "spark?" Welders spray sparks, as do fireworks: is it a part of the material that burns and then is vaporised?

John Martin, Hong Kong, China

• Actually, when a spark is thrown from a welder or a grindstone, it does not, as the questioner suggests, vaporize. Rather, a spark from these items is a red-hot speck of metal, which rapidly cools due to its lack of mass. Similarly, a spark from a firework is a particle of red-hot powder ejected from the firework container.

  Paul, Burnsville, MN USA

• If you've been given a birthday ice cream with a sparkler sticking out of it, you'll have seen the flecks all over the ice cream left by the bits of burning iron. Not very appetising.

  John Davis, Otley UK

• Yes, both processes work in a similar way. Fireworks are burning metals which produces light. Sparks from a welder are slightly different. They are small bits of metals blasted out from the point of contact between the two metals (in electrowelding). However, they are not meant to burn and produce light. The light you see is radiation emitted from the small bit of metal because of its inital high temperature. And no, its doesn't vaporise. It disappears because it stops glowing after a while, and if u observe the floor carefully, there are visible bits of ashy things or bits of metals.

  John, Singapore

• Sparks associated with electric current in air, such as lightning or the effects seen in B-movie sc-fi scientists labs, are caused by the air molecules themselves being raised to such a high energy state as to emit radiation in the visible spectrum. I'm not sure the air can be said to burn (as burning is rather a complex chemical reaction), but I believe it's a similar principle as light being given off by anything that is burning, such as the welder's sparks.

  Phil, London