Q. Why is a small space left at the joint between the two rails?

A. Metals expand on heating and contract on cooling. A small space is left between each set of two rails of the railway line to allow for their expansion in summer and contraction in winter, respectively.

Q. Why can’t a petrol fire be extinguished by pouring water over it?
A. Water, being heavier, slips down and petrol will rise to the surface and continue to burn as before. Hence, water cannot be used for extinguishing petrol fire.