Johannesburg - Most people know who Albert Einstein is, or have some vague idea of the scientist with the wacky hair.What people have a hard time understanding are the theories that made Einstein a household name.According to Sheldon Herbst, from the University of Witwatersrand, this is because most of us are locking into a way of looking at the physical things in our universe based on the nearly 300-year-old theories of scientist Isaac Newton. People also stay away from Einstein's theories because they are scared of maths."We are trying to bring the ideas of relativity to the general public," he said at an astronomy night lecture at the university's education campus."Until we can speak to an 8-year-old and tell her that space-time is unified, and she doesn't look puzzled, then we have not been freed from our Newtonian minds."Here are four ways of understanding one of Einstein's ground-breaking theories.1. There are two types of relativityOne, called the general theory of relativity - says that space and time are actually different aspects of the same thing - and that this space-time can be warped and bent.The other, the special theory of relativity, deals with how things look different to people in different places, or when moving at different speeds. 2. Special relativity says the speed of light is always the same, no matter how fast you are goingIf you are travelling at 120km/h in your car at dusk, and you switch on the car's headlights, those lights are not travelling at 120km/h plus the speed of light. Their speed is always "the speed of light". This sounds counter intuitive, but the simple fact is that the speed of light is constant and never changes.3. Time moves differently for people observing it differentlyIf you are playing a game of table tennis at the roadside, and you bounce a ball twice in the same spot - it's pretty obvious to you that the ball bounced twice in one spot. But if someone else is playing a game of table tennis on the back of an open truck, and they bounce the ball twice on the same spot as they drive past you, to you it looks like the ball bounced in two different places because the truck is moving. To the person on the truck however, the ball bounced twice in the same spot.This can apply to how time is measured. If it takes one second for the ball to bounce twice - the player on the truck's one second will seem longer to you on the roadside because according to you the ball had to cover more distance in its bounces than yours.Einstein once told his secretary when she was bothered by inquisitive interviewers, who wanted to know what relativity really meant, to answer:“When you sit with a nice girl for two hours you think it’s only a minute, but when you sit on a hot stove for a minute you think it’s two hours. That’s relativity.”4. The theory of special relativity is the stuff of science-fictionThe twin paradox is a thought experiment used by scientists to think about special relativity. Let's take a pair of identical 20-year-old twins. One is put onto a rocket and sent to our second nearest star, Alpha Centauri, almost at the speed of light.The other sits at home and bides her time. When the one in the rocket returns back from her nearly eight-year round trip - she will be eight years older, but her twin sister will be a pensioner.It might sound like science-fiction, but according to special relativity, this is entirely possible.