It’s a chilly day and I have a candle sitting on my desk as I work. The bright sunlight throws a silhouette of the candle onto the wall and it shows the shadow of something emanating from the top of the candle – the shadow behaves in a similar way to a vapour of steam or smoke as it rises. But when I look above the candle I cannot see anything – there is no smoke or steam visible to the naked eye without the benefit of a shadow. What am I seeing?

3 of us tried to quit smoking while working at a typewriter repair shop where we used strong chemicals to clean the machines. For some reason the smell of the chemicals made us laugh uncontrollably. Only while we we're trying to quit. None of us ended up quiting and when we started smoking again, no more laughing like crazy Why did that happen?