Eggs do not start out egg shaped. While they are forming inside the chicken, they are very round, and they don’t have a shell. When the egg is almost ready, the chicken pushes it towards the end of the oviduct (the tube that eggs travel down to get out of the chicken). The squeezing makes the egg into an egg shape. While it is being pushed, the egg grows a shell around itself!
Luckily, the egg shape has a lot of benefits. The top of an egg is a small dome shape, and the bottom of an egg is a larger dome. Domes are very strong shapes. They distribute force from the top evenly throughout their structure. The strength of the shape means that the material used to make a dome doesn’t have to be very thick.
Eggs are very strong if you push on them from all directions evenly because they are made of domes. Chickens are soft, and nests are soft, so the pressure of the chicken is even over the whole surface of the egg. This means chickens can sit on eggs without breaking them. If eggs were a different shape, the shell would have to be a lot thicker or they would break when the chicken sat on them. If eggshells were thicker, the chicks would have a hard time breaking out of them when they hatch.
Eggs also have another interesting property. They can roll, but they don’t roll away very far. Eggs tend to rest a bit towards their pointy ends. They are wider towards the round end and skinnier towards the pointy end, so the egg acts like a cone, and rolls around in a circle. This way, if an egg gets bumped, it doesn’t get lost, or fall out of a nest in a tree.
Birds that lay their eggs in holes tend to have rounder eggs than those that lay them in nests in trees or on top of cliffs. Eggs in a hole don’t have anywhere to roll away to, so they don’t need to be as pointy.
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