double spending , you can create money out of thinner air
To double spend BTC, you need to make one transaction to the person meant to receive the money, and one transaction to an address you control yourself. This would typically be another address in your wallet. The two transactions would spend the same bitcoins, but send them to different addresses. Those transactions would look like completely normal transactions.

Now, you will not be able to create any new bitcoins. Because the same money is spent twice, only one of the transactions can be accepted into the blockchain. That's why one should wait for confirmations before accepting BTC as payment, as there's no guarantee for which transactions will be accepted in the next block and which transactions won't.