How do I link to cells in other sheets?

I’m using Numbers for iOs. How can I get a cell in one sheet of a given spread sheet to reference the value of a cell in another sheet?

I am assuming both sheets are held within the same spread sheet.

In this case one sheet, Office, lists a series of expenses for my office. I want that total to be read by another sheet of expenses, Total Expenses, that takes the totals from other sheets, and summarizes the totals from other, more detailed lists.