The SIM card, or subscriber identity module, has been a standard in GSM phones for many years. It's always been very easy to switch your mobile network account between GSM phones since all you had to do was take the SIM card out and put it in a different phone and everything worked because all of the SIM cards and SIM card slots were the same. Unfortunately we're starting to see some manufacturers implementing a smaller SIM card size called Micro SIM. The advantage to the Micro SIM is that you can have a slot in the phone that's a few millimeters smaller than what you would need to support the standard SIM cards. The disadvantage is incompatibility.
In the video below we show you how to cut your standard-sized SIM card to fit into the new Micro SIM slots and also how to use a SIM card adapter to get that newly cut Micro SIM card to go back into a standard GSM phone. Some people have used scissors to simply cut down the normal SIM card to work in a Micro SIM slot, but that can be difficult and inaccurate. Of course you could buy a Micro SIM card from your carrier as well. I found the best way is to buy a specially designed Micro SIM Cutter, many of which come with 4 adapters that let you use a Micro SIM in normal SIM slots. You should be able to find one in the $2-16 range. Within a couple seconds, the Micro SIM cutter makes your normal SIM card compatible with Micro SIM phones.
Unfortunately there's a gotcha with the adapters when you want to put the Micro SIM back into a normal phone. The adapters make the whole thing ever so slightly thicker and that makes inserting and removing the Micro SIM card much more difficult. To remedy this you can use any nail file to sand down the back of your Micro SIM card in order to reduce its thickness. Keep looking at how the Micro SIM fits in its adapter as you sand it down until it's completely flush. Once you get it just right, you'll be able to switch between Micro SIM phones and normal SIM phones without too much frustration.