I cannot get a count down GUI to refresh when a script updates the int value, what should I do?

I am making a count down, but when I touch a part, and a line of code gets set off. Just saying:

workspace.time_left.Value = 30

Then, a count down get set off. But I cannot get the count down GUI to update. It counts down when I start game, but not when I touch that part.

local text = script.Parent
local left = workspace.time_left


while true do
	wait(.5)
	if left.Value > 0 then
		text.Text = "Intermission: ".. left.Value
	else
		workspace.shell.Parent=game.ServerStorage
		text.Text = "Game is starting!"
	end
end