data is in sheet 2 cells A1:A20
Name that group of cells by highlighting them and then in the3 cell
reference box
(where you would see the name of each cell e.g A1 ) type in a name.
That group of cells can now be reference by the given name from any
sheet.
Now go to the cell in the other sheet you want to create the drop
menu.
On the menu bar select Data - Validation
For the Allow criteria select List
For the Source type in
=the_name_you_gave_your_grouped_cells