Is it possible to detect what part is hitting another part?

You will need to use a Touched event, passing the part called "Back" as the target of the function parameter, using an "if" condition, like this:

local one = workspace:WaitForChild("One") -- Your "One" part.
local Back = workspace:WaitForChild("Back") -- Your "Back" part.


Back.Touched:Connect(function(Part) -- "Part" is the parameter what we want to be "One".
   if Part.Name == "One" then -- if "One" hits "Back" then:
         print("It is One!")
         Back.Transparency = 1
   else
      print("It isn't One, it is".. Part.Name.."!")
      end
end)

print("ola mundo da lua!")