Switching Between Open Workbooks In VBA

I have three workbooks named macroWorkbook, databaseFile and dataFile. All of my programming resides in macroWorkbook. The VBA stuff in macroWorkbook open the two other workbooks. The names of the other two files are databaseFile and dataFile have date suffixes affixed to keep weekly data from crossing. How do I programmatically switch between databaseFile and dataFile? Eventually I want to take the contents of sheet1 in dataFile and place it at the bottom of sheet3 which already has data.

I thought about opening dataFile > copy the contents of from sheet1 > closing dataFile > open databaseFile > paste. This turns out to be more complicated and so I'm here looking for some assistance.

I am running Excel 2010 on Windows XP.

Tony