This is because balance, or the sense of level, is determined by several mechanisms. One of these mechanisms consists of a hollow organ (the sem-circular canals) in the vicinity of the ears filled with tiny hairs (cilia) filled with fluid that sort of sloshes around. When you rotate like a top you cause the fluid to slosh. The fluid hits the hairs which send back a message that you are not level. Under certain conditions the sensations can make a person believe he/she is upside down, in the absence of visual confirmation. This is why pilots must learn to always rely on their instruments, regardless of whether they feel the plane is diving, turning or climbing. During flight training pilots are blindfolded and places on a rotating chair and asked to turn their heads in a certain way. Finally the chair is stopped and the pilot lurches foward, convinced he is being pushed off the chair.
thanks you so much, i finally understand why ...i asked my biology teacher this question but he didn't know , i will give him this answer too , :)