Why Is There A Round Hole In The Middle Of The Chair Mould?
Remember the physics experiment "Magdeburg Hemisphere Experiment" in junior high school? The internal vacuum causes a strong force to pull the hemisphere apart. Although there will be no vacuum in the middle of the stacked stool, the air pressure in the internal cavity will decrease before being pulled apart, creating a similar effect.
To eliminate the effects mentioned above, any form of "hole" can be opened. Why is it "a round hole not too big or small"?
First of all, opening a square hole will be very tragic: similar to why an airplane hanging window is a rounded rectangle, a square, or any hole with a sharp corner, the stress on the sharp corner will be relatively concentrated, and it will easily crack when subjected to force.
Among other holes with rounded corners, the round shape is the easiest to mold, so choose round holes. So there will be a round hole in the middle of the chair mould.
Why is it "one"? To be breathable, if the size is similar, one is enough.
Why is it "not big or small"? If it is too large, it will affect the strength of the structure. If it is too small, the fingers will not be able to get in when you hook the upper stool!