I've found a simple method online for creating a secondary dropdown
list based on the selection from a primary list, but it requires use of
two worksheets. 1) Why is that? 2) Is there a another simple way
that can be done all in 1 sheet?

3) Finally, is there a way to manually type in the data validation box
what you want, i.e.
=if(item_from_primary_list
then "secondary_list_item_#1", "secondary_list_item_#2...) ??
Thanks!