heat and heat transfer? and the faster the molecules are moving the warmer it is, and your molecules are moving faster than the desk, and when you put your hand on the desk (everyone put their hand on the desk/wall/whatever now) your molecules are bumping into those of the desk (here you can bump into a desk to transfer motion as an illustration) and they start to move faster and....and if the molecules move faster and faster and eventually the molecules are moving so fast that they just can't keep to themselves and the spit out bits of themselves and that becomes light and....

I hope you do really really well, regardless.