If you had 10 children, and each one of your children had 10 children, and this practice continued for 10 generations from you, how many descendants

10^10 = ten billion at the bottom level of the tree;
then 10^9 at the 2nd from bottom level;
then 10^8, 10^7 etc etc.

Adds up to 11,111,111,110

Ends in a zero because you don't count yourself among your decendents.