assigning value to specific index leading to change in value at all indexes

Question

Why on assigning value to a specific index in 2d array of python, changes all the index with the value?

def binomial_coff(n, k):
    inter = [[-1]*(n+1)]*(n+1)
    for i in range(n+1):
        inter[i][0] = 1
        inter[i][i] = 1
    print(inter)
    for i in range(2, n+1):
        for j in range(1, i):
            inter[i][j] = inter[i-1][j-1] + inter[i-1][j]
            print(i, j, inter[i][j], inter[i-1][j-1], inter[i-1][j])
    print(inter)
    return inter[n][k]

I had tried the block in jupyter notebook and repl.it