Why importing Numpy creates many processes

I got no proper answer after looking up for some time. I am using a processor with 4 cores; Linux, python 2.7. While importing Numpy, I see 4 processes are created. Although only one of them is doing all the work, which is in fact the process with pid same as when I do os.getpid() from the python script. Why the other 3 are created? Is there any way to prevent from spawning? For a quick test, I did this:

import numpy as np

raw_input() # just to pause

size = 1000000
a = np.random.randn(size)
b = np.random.randn(size)

while(True):
    c = a * b