"The Varnish Principle" joke
Once little Siripala was playing with sand in the garden, and his grandfather was watching him. Then suddenly he pulled out a worm from a tiny hole in the sand. The grandfather saw this and said, "I will give you ten rupees if you put the worm back to the hole". Siripala tried his best but the worm was not stiff enough to put into the hole. Then he brought a tin of varnish, and applied some on the worm, and left it for few minutes to dry. When the worm was dry, it became stiff, and then he easily put the worm into the hole. Siripala's grandfather said, "You did it boy! I'll give you your ten rupees tommorrow".
The next day the grandfather gave him twenty rupees. Siripala asked, "Why did you give me twenty?". Then the grandfather said, "Well; The extra ten rupees is from your grandmother, as an appreciation for your invention".
Not enough votes...