Strychnine is a strong poison that comes from the seeds of a tree. A very small amount kills, and it kills in a terrible way — it makes all the muscles pull tight at once, so the body stiffens and arches. Because of this it was famous in the past as a murder poison, and you will meet it most often in crime stories and detective novels; Agatha Christie used it in her very first book. In real life it has mainly been used to kill rats and other animals, though many countries now control or ban it for that too. It has also been given as a medicine in tiny doses — old tonics contained it, and athletes once took it as a stimulant — but this is history, not current practice; doctors do not use it now. Two notes on the word itself. The spelling is odd for English and the pronunciation surprises people: the ch is a hard k sound, and the final part sounds like 'neen', so roughly STRIK-neen. And it is uncountable — you say 'strychnine' or 'some strychnine', never 'a strychnine'.