Generating randomness

Ethereums Virtual Machine is determinsistic - this means that if you replicate an operation thousand's or hundreds of thousands of times across the network on the machines of miners they will each produce the same outcome. This is necessary as without these characteristics it would be impossible to verify if a block mined is valid. As such it precludes the use of random or pseudo-random number generation in our contracts code - this is a problem as random number generation is an important part of many programs.

Fortunately During the proces of mining blocks miners do produce some randomness - the hash of the succesful blocks headers is randomly generated each time a new block is produced.