Impossible means “I aM Possible”
Do you believe that man can walk on the moon? It is simply not possible few hundred years ago but now it is a pretty common thing that every knows that man had achieved walking on the moon already. Don't always put a “NO” towards any situation that is challenging for you as there might be ways to tackle the situation where you might not know yet.
Try to see things in a different perspective and you might be able to discover something that you have not yet explore before.