give me a lesson , do not leave your coin in exchange
Unfortunately it's a catch 22, to use exchanges, you have to put coins into them... but then they are at risk.
The problem here is not that we put coins on an exchange, its that some people take those exchanges as wallets. They let their coins on the exchange waiting to maybe trade them. Those coins shouldn't be left online. The coins you trade are at risk but if you don't want to take this risk, don't trade.