Sowing your grain among stones, where they cannot take root, in the water, or on sand. «In aqua scribis, in harena ædificas», writing on water, or building on sand, with many others, are phrases used by the Romans, and are applicable to persons bestowing much labour in effecting what is impossible to be done, or heaping favours upon an ungrateful person, from whom no return can be expected. «Can the Æthiopian change his skin, or the leopard his spots».