Overide the data in list box?

Hi all,

I have some list boxes in my excel sheet,
I want to make users to select data from this list dropdown or enter thier own data.

Right now when user enters other data than ones which are in the dropdown ,excel pops an error.
I dont want this to happen,

How can i do this?

Thank you,
Glen

ya roy, I am using data validation.

and selecting list option in that.