When you are cutting onions, there is a volatile substance, so you often get weepy. Typically, the method used to prevent this case are as follows. Firstly, before cutting onions, you can make kitchen knife dip in cold water for a while, and then you will not suffer the volatile substances which stimulates tears. Secondly, cut the onion in half, soak the cold water and then cut, maybe it is ok. In addition, the onion volatile substances by nose stimulus the eyes. So long as you make your nose stuffer with tissue paper, you also can stop tearing. Of course, there are many methods to help you, but relatively more complex, so they are not put forward here.