That is called time decay of options. Remember, options prices have two components viz. intrinsic value and time value. If you buy a Tata Steel 650 call option at Rs.20, when the CMP is Rs.658, then Rs.8 (658-650) represents the intrinsic value and the balance Rs.12 represents the time value. It is the time value that decays because when there is no time left there is no time value left. That means it is not all that easy to make money by buying call options since risk is limited but the time works against you towards expiry. It is interesting to note that over 90% of all options are known to expire worthless. So, options are actually weighted in favour of the seller. That means you have to be very careful that you can better your breakeven point of the option.