A general rule of thumb is that if your economic model has no value judgements, it's positive economics, and if it does have value judgements it's normative economics, since to tell someone what he ought to do, you first have to judge what is best for him.
A subscriber identity module or subscriber identification module (SIM) is an integrated circuit that is intended to securely store the international mobile subscriber identity (IMSI) number and its related key, which are used to identify and authenticate subscribers on mobile telephony devices (such as mobile phones and computers).
your SIM card is your memory card which stores phone numbers photos and info about your phone whhen your phone tells you to check your SIM card all you have to do is locate it (usually under the battery of your phone) turn your phone off, take it out and reinstall it then turn your phone back on it should work fine.