The easiest way to earn bitcoins for me is to purchase tokens during the ICO. Usually, such tokens can grow tens of times after going to the crypto-exchange.
yes, this is one of the best ways and provides many advantages as a coin holder of one of the projects. or you can sell the bonus coins you get from ICO.
The OP is asking how to earn bitcoin without investing any money on it. How can he invest in these ICOs or projects if he/she don't have any money to invest with? If you want to do something that will make you earn some digits, giving some services or selling off your skill for it.