Why do the plugs on some appliances have two prongs and others have three prongs

hi moneyminded some big appliances have much more electricity surging through the lines so have a third prong to ground the outside so you do not get a shock that could even kill you if it was powerful enough. Be glad of that third plug on all your bigger appliances. our computers all have that too. plus an additonal power surge thingy that goes with the computer.