 Now imagine a four-dimensional object, cut into three-dimensional timeslices. In a non-relativistic world, there’s only one way to do this, because there’s only one way to divide up events into groups of simultaneous events: it’s like cutting the cake at right-angles. But, by relativising simultaneity to reference frames, STR gives us lots of different ways to cut the four-dimensional object into three-dimensional slices, just as there are lots of different angles at which to cut the cake. The shape of the three-dimensional slices you get by cutting in different ways, from the perspective of different reference frames, can all be explained by the shape of the four-dimensional object you’re cutting up.