private void DataGrid1_ItemDataBound(object sender, System.Web.UI.WebControls.DataGridItemEventArgs e)
{
     e.Item.Cells[0].ToolTip = "Column0";
     e.Item.Cells[1].ToolTip = "Column1";
     e.Item.Cells[2].ToolTip = "Column2";
     //....other cells like above
}