The Python – for loop only loops 3 times. Why?

I wrote a deceptively simple loop. There are 6 items in the list, and it should loop six times. However, it only loops 3 times. Why?

list1 = 'one two three four five six'
newlist = list1.split(' ')
print (newlist)
list2 = ['seven', 'eight', 'nine', 'ten', 'eleven', 'twelve']

for number in list2:
    nextnumber = list2.pop()
    print ("Adding number ", nextnumber)
    newlist.append(nextnumber)

print (newlist)