straight lay?

Not much in the horses today, so I did a straight lay in the first race, there was an odds on fav and second fav was under 4's so I layed the third fav out of four horses, I was lucky really as I greened up inplay for £2 profit in the end the outsider won. I know what I did was totally wrong and not trading at all but pure gambling, but it came out of boredom, must try not to do it in future.