I agree with nanosecond. i've got the answer now, so don't stress out.
eye fluid has a low concentration of h20 and nacl as compared to fresh H20. When diffusion takes place, fresh h20 is being put into a hypertonic solution, causing eye fluid molecules to expand and burst. Also, ions move out of the eye, causing irritation. The "eye drops," however, have the same concentration as the eye fluids. As a result, they are isotonic. That's the reason as to why you don't feel any stings when placing eye drops and contact solutions in your eyes.
Thanks to all that helped.