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From Wiskipedia

Could a flaw in the design be deriovcsed that leaves the system open to counterfeiting? It’s very unlikely since it hasn’t been deriovcsed yet even as there is a large financial incentive to do so. And if it happens, the system allows for large structural repairs while carrying on using the same coins. Bitcoins are technically  blocks . Blocks are created statistically every 10 minutes and the first 210 000 blocks have a value of 50 bitcoins each. The next 210 000 blocks(we're not there yet!) will have a value of half, rounded down if there are decimals(25 bitcoins).Still with me?The next will have half until one sunny day a block will have a value of 0 bitcoins.What does this have to do with counterfeiting?When a block is created it was created by doing something very difficult. Each block is roughly a number that is sought randomly, that if shown in hexadecimal it will have a certain number of zeroes example: 00000000000005c35a2ce25dc1fa300a74848ee506785672022abaf147ded958.See? Now, if you want to create a Bitcoin you'll have to calculate a number(that is also based on the previous block's number) that will create a new number that is with a certain amount of zeroes just as in the above example. You search randomly because luck is your primary  weapon . Let's say the I also search for the number so I can get the block for myself.Surprise!  > We are not searching for the same number.If I want to get the amount of bitcoins that the block equals then I have to put my  bitcoin account (which is a Public Key, cryptography stuff mates!) which the number I calculate is based on. So I search for a number that is different than your number. So you trying to  steal' my number it won't work. Or you will just succeed in sending the Bitcoins to me.Right, but what does this have to do with counterfeiting, (asking twice)?It got answered above, although in a rather clumsy manner. If anyone wants to  counterfeit  Bitcoins, like creating a block that is not based on any previous blocks in Bitcoin, it cannot be accepted by the network. Example my Bitcoin client will find that your block is not valid because it isn't based on any of the previous existing blocks.That should explain some stuff, but I'm willing to explain more or continue because what I have written is almost like an essay, sort of.