Fairly often, you'll have data in a cell that you just want to get rid of. You should click on the cell, press the delete key, and press Enter. Don't ever click on the cell and then press the space bar!!!. This looks as if it's doing the right thing because the cell goes blank, but there is "hidden" data in there -- that one space -- and that's enough to confuse some aspects of the program.
You can also delete the contents of a whole range of cells by selecting them with the mouse and choose Edit:Clear from the menus.