may be a dumb question,but I can't understand why?

[luckey]

the following function is in bpnn.py
def makeMatrix(I, J, fill=0.0):
    m = []
    for i in xrange(I):
        m.append([fill]*J)
    return m

I rewrite it to :
def makeMatrix(I, J, fill=0.0):
    m=[[fill]*J]*I
    return m
the result is defference
Can someone tell me the defference between the two function?