Dynamically creating textbox..

Question

Hi,friends..
i have one table it have 5 column...in the aspx page i have one text box and button when the user click the button i want to add another textbox dynamically..
but it can allow only 4 textboxes because i have only 5 column in table...another one important poin is suppose user genarated only three textboxes means when the user click the update button i want to update the table with the three values at the same time the remaining two column automatically update with null value..
How can i achieve this...
thanks in advance....