an old black-board which is not so 'black' nw...

when a black-board is new its completely black.....people writes on it.....thn erases it....thn again writes on it.....erases it......lik tht it goes......it will b used lik this for a long time......nd people will c tht tht black-board is still usable as one can read whatever is written in it......but if u go very close to it and looks at it (ppl with dust allergy will suffer) u c tht its nt tht black as it woz b4....the erased dust will b coverin it on most parts...some times even the writin space will b left with traces f old writings...but no one notices tht...the corners will b havin dust particles...... once a part f a meaningful writing which now does not have any meaning in bein there....and it remains there for a long time.....some times till the black-board is thrown out....THN THERE COMES A SOLUTION--- INSTEAD F BLACK BOARD Y DONT WE USE WHITE BOARD!!!!!!!...............