Mathew 17:20

And Jesus said to them, because of your unbelief: for verily I say unto you, if you have faith as a grain of a mustard seed, ye shall say unto this mountain, remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you.    

A mustard seed is a small seed with great potential.  Never does it say, “Well, I am too small to grow into a great tree.”  The mustard seed, when put in the ground, just grows.  When we believe that we have the same seed of potential, nothing will be impossible for us.  The Holy Spirit resides in the believer to develop that potential, just believes, and don’t worry how the water will come or whether it will be sunny, only grow.