1. First stroke:Straight from the middle, dividing the cake in two equal halves
2. 2nd stroke: Similar cut at 90 degrees, so that cake is now in 4 equal parts
3. 3rd: From the side throughout the cake ;) (you will need a knife longer than cake)
Cut the cake into two halves. Put one half on top of other, cut again. Now again the put 4 pieces on top of each other. Cut the piled up pieces from top in half again. You get 8 equal pieces with 3 blows...