What happens if you put diesel in a petrol engine and vice versa?

 What happens if you put diesel in a petrol engine and vice versa?

Diesel in petrol engine - If it was a mixture of the 2, and there was enough petrol in the mixturethe engine would run, albeit not great, and put out masses of white smoke from the exhaust like James Bond. This is the unburnt diesel being vapourised in the hot exhaust pipe. Pure diesel in a petrol engine just won't run. 

Petrol in diesel engine - % of diesel in the mixture needs to be higher than in the previous scenario, and if it runs, it will run like a 3 legged cow in a sand pit! Diesel engines hate petrol and are less likely to be tolerant of cross contamination than a petrol engine. 

Why?- Petrol engines work by the vapourised fuel and air mixture being ignited by the spark from a spark plug to cause the explosion. Diesel engines do not use spark plugs, they compress the fuel / air mixture to a much higher pressure, which in itself creates the necessary heat to cause the explosion. That's why you can't run the different fuels in the wrong engines!