How do you gradually increase player transparency?

Hey Sparking!

So, with your loop, you're editing part.Transparency to be the same number over and over again, since t isn't being changed in your loop.

local touching_part = script.Parent
local debounce = false -- A debounce is useful for keeping your touched event from running more than once at any given time and forcing your fade to go faster

touching_part .Touched:connect(function(part)
	if(debounce == false) then
		debounce = true
		local t = 0
		if(part.Parent:findFirstChild("Humanoid")) then
			for s= 1, 10 do
				part.Transparency = t + 0.1
				t = t + 0.1
				wait(0.5)
			end
		end
		debounce = false
	end
end)

I'd also recommend that you search for the humanoid using findFirstChildOfClass, in case your humanoid is named something different. Also, you can use your for loop to change your part's transparency a bit more easily, like so:

local touching_part = script.Parent
local debounce = false 

touching_part .Touched:connect(function(part)
	if(debounce == false) then
		debounce = true
		if(part.Parent:findFirstChildOfClass("Humanoid")) then
			for s= 0, 1, 0.1 do
				part.Transparency = s
				wait(0.5)
			end
		end
		debounce = false
	end
end)

Avoiding HumanoidRootPart's transparency being changed is a piece of pie. Just add to your if-statement.

local touching_part = script.Parent
local debounce = false 

touching_part .Touched:connect(function(part)
	if(debounce == false) then
		debounce = true
		if(part.Parent:findFirstChildOfClass("Humanoid") and part.Name ~= "HumanoidRootPart") then
			for s= 0, 1, 0.1 do
				part.Transparency = s
				wait(0.5)
			end
		end
		debounce = false
	end
end)

There is, however, a problem with this particular solution: With the debounce there, you can't fade more than one part at a time. So then, how do we fix this? Well, there are a couple of ways to do so, but a table will suffice.

local touching_part = script.Parent
local debounces = {} 

touching_part .Touched:connect(function(part)
	if(not debounces[part]) then -- Check if the part is being faded
		debounces[part] = true -- If it's not, go ahead and add it to the table
		if(part.Parent:findFirstChildOfClass("Humanoid") and part.Name ~= "HumanoidRootPart") then
			for s= 0, 1, 0.1 do
				part.Transparency = s
				wait(0.5)
			end
		end
		debounces[part] = nil -- Get rid of the debounce for this part
	end
end)

Hope this helps! :)