My Magic Cookie number is: 22735

In the Edit Object Definition screen in TMog, you can see it written in hexadecimal (hex) as 58CF

But ...
... what if the TMog site gave me 102335 as a Magic Cookie number.
There is only space for 4 digits in the GUID number, so TMog has chopped off the first digit.
THIS would be the actual Magic Cookie number that my objects would have.

So ...

... be aware of the problem because if two (or more) people have the same Magic Cookie number, their objects could have the same GUID numbers, and only one of those objects will show up in the game. It might be your objects that will not show up, or it might be yours which knock out other sites' objects, like 7DeadlySims or Well Dressed Sims or Sim Freaks or Cheap Frills or ... BunnyWuffle's!!

Any Magic Cookie number that is greater than 65535 will have had the 'chop' treatment in TMog. If you have one of these bigger numbers, perhaps you could work out what your number really is and check at Sandshifter's site or the database at the MagicCookies eGroup to make sure that it is not the same as anyone else's — not everyone is listed, but it gives you a start.
For PC'ers: you can do the conversion from decimal to hex with your computer's calculator. Put it into scientific mode and enter your decimal number. Click on the Hex button and you will see your number change to hexadecimal.

For Macintosh users: if you are running TMog, you'll be using either VPC or a real PC. However, for the curious, you can see the GUID number with IFFSnooper - the last two digits followed by the first two digits make up the Magic Cookie. There are free binary calculators at Tucows or Shareware.com.
<-- Note the hex has 5 digits.
An explanation of Magic Cookie numbers —
particularly important if your number is higher than 65535

You can also see the GUID number of an object and your Magic Cookie number in SimCategoriser. On the 'Edit Object' screen, double-click on the thumbnail picture — but note that the Object ID (GUID) number is written in reverse order.

Every Sims object has a unique ID number which the game uses to keep track of everything. If you are making objects to share with others and to minimise the risk of ID clashes, it is important to have a Magic Cookie number (just one, for you the creator) which TMog embeds in your object's ID number.
It doesn't matter if you don't know how to count in hexadecimal. It's just one of those horribly logical computery things that most people in the world can happily ignore — including me!