how to count from 0 to 15 in binary.(Activity 5[February 9, 2010])

0= 0
1= 1
2= 10
3= 11
4= 100
5= 101
6= 110
7= 111
8= 1000
9= 1001
10= 1010
11= 1011
12= 1100
13= 1101
14= 1110
15= 1111