Well, most people have always wondered why the earth is spherical and people don't fall of the southern hemisphere or stand upside down.
This is because, up or down is actually relative to an observers position. For instance, a man in Wellington new Zealand will take the southern hemisphere as up and the northern hemisphere as down while a man in Madrid Spain will take the northern hemisphere as up and the southern hemisphere as down.

Why don't we feel upside down on either halves of the poles, or feel sideways around the equator?

Well, this is because man evolved in the presence of gravity hence our sense of up, down or sideways are all functions of the force of gravity acting through us. Hence, if a man stands south of the earth sphere, gravity acts downwards towards the ground or better still center of the earth, if he stands north of the earths sphere gravity still acts downwards towards the center of the earth sphere. Our geographic north and south are only adopted by convention; outside the planet there's no fixed north and there's no fixed south.
Thank you all

Good. any doubts will be treated after the next millennium!