This is one whole because the fist number is 1 but then the next number is 0 so it is a whole. 
This is the number l chose. The first number is 1 so that means that it is 1 whole then there are two 0 so tht means that there is nothing representing the tenths and hundredths so then the next number is is 1 and it is in the thousandths. l now that is right because you need 1000 to make a whole.
Great work, Milly!
Your last sentence explains why the little block is equal to one thousandth.
Thank you☺☺