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Many have taken Sod's Law as the root causal factor for why toast always falls butter side down. This is incorrect. The real answer lies in simple physics. A falling piece of bread is influenced by several factors. The major one is gravity. The second and often overlooked one is height of the standard kitchen counter. The third factor is the size of the bread. The fourth and final factor1 is the angle at which the bread initially leaves its starting position. Most falling bread falls either to the left or the right. Rarely does a person hold both sides of the bread level with the ground and simultaneously drop it from both hands at the same instant. Instead, one is usually balancing the aforementioned bread slice in one hand and a bread knife in the other.
A Period of Rotation
The angle imparted by the ungainly divestiture of the bread causes the bread to rotate as it falls, fuelled somewhat by the uneven mass distribution of the butter or other viscous substance (ugh! like Marmite) on the bread or toast slice. This rotation has a period and unfortunately the floor intersects this cycle exactly at the half way point, resulting in the deposition of the bread slice with the viscous side down. If the counter top were twice as high, the bread would land butter side up 95% of the time. You can check this with a ladder in your kitchen and drop bread missiles from it. If you haven't a ladder handy, cut the bread in fourths or use a saltine cracker and drop from normal counter height. You can see that the cracker has a different period of rotation due to its smaller size, and should be able to complete a full rotation before hitting the floor.
Do not ask your spouse for permission to conduct this experiment in your kitchen. Remember that it is better to ask for forgiveness than to ask for permission.