add rows in a table on demand.

I pasted a table in design mode as a place holder. I want to create rows on demand when a user clicks a button but all I see is one row. WHY?

protected void Button1_Click(object sender, EventArgs e) {
DropDownList ddl = DropDownList1;
TableRow tr = new TableRow();
TableCell tc = new TableCell();
Label l = new Label();
l.Text = ddl.SelectedValue;
tc.Controls.Add(l);
tr.Cells.Add(tc);
Table1.Rows.Add(tr);
}
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