How can I conditionally hide or display data in a cell?

[mrsphyl]

I would like to display a cell dependent on the value of another cell.

If the value in a cell exceeds 10% of the value in another cell, I would
like to display a warning message in red. I don't want to see the message at
all if the value in the original cell does not exceed 10%.

 

[Bernard Liengme]

You would need a third cell for a message
You could colour the target cell using Format | Conditional Formatting
Something like--- Formula Is A1>0.1*B1
best wishes

 

[Spiky]

You would need a third cell for a message
You could colour the target cell using Format | Conditional Formatting
Something like--- Formula Is A1>0.1*B1
best wishes

Just to add:
Type your warning message in the 3rd cell. Color the font white to
hide it normally. Then do a conditional format of red font with the
formula Bernard gave you.