on Gridview1_PageIndexChanging grid Should not Reload..

Question

i have check boxes in last column of gridview1, when i check some checkBoxes of page index 1, and go to 2nd index to select some more items, and come back on 1 st index, i don't got any check box selected...
.
i don't want this.
how to prevent this,
i want that check boxes, remains selected when back to page index..