How to make chat door answer to multiple answers?

• Declaring variables local is best practice (due to how Lua works it is slightly faster). This applies to functions, too. Just note that you can't reference a local variable until after the statement is complete, so if you have two local functions that need to reference each other, you need to "forward declare" at least one of them. Example:

-- Wrong way
local function first(callSecond)
	print("In first")
	if callSecond then
		second() -- "second" is declared below and so isn't available here
	end
end
local function second(callFirst)
	print("In second")
	if callFirst then
		first()
	end
end

-- Right way: forward declare 'second'
local second
local function first(callSecond)
	print("In first")
	if callSecond then
		second() -- "second" is declared below and so isn't available here
	end
end
function second(callFirst)
	print("In second")
	if callFirst then
		first()
	end
end

• PlayerAdded is recommended over ChildAdded
• connect is deprecated; Connect is recommended instead
• You can use a StringValue to support multiple answers but a table is probably easier to edit in this case:

local answers = {
	["cat"] = true,
	["dog"] = true,
}
local door = script.Parent
local decal = door.Decal
function onChatted(msg, recipient, speaker) 
	msg = string.lower(msg) 
	if answers[msg] then
		door.CanCollide = false 
		door.Transparency = 0.7 
		decal.Transparency = 0.7
		wait(2)
		decal.Transparency = 0
		door.CanCollide = true 
		door.Transparency = 0 
	end 
end 
game.Players.PlayerAdded:Connect(function(plr)
	plr.Chatted:connect(function(msg, recipient) onChatted(msg, recipient, plr) end) 
end)