Smokeydragon

Please help me out .... i managed to solve all the steps, but how on earth do you solve step 2?

There is a water lilly in a pond. Each day it doubles in size. On day 10 the lilly fills the whole pond. On which day is it halfway?

Lu - as there are no more vowels left except for the "y", I hope they stopped having children.

It is riddles like this that can make one really feel stupid!! I don't have the faintest idea ..... any geniuses out there that can help?