The Stamina bar script!

local player = game.Players.LocalPlayer
local character = player.Character

local UserInputService = game:GetService("UserInputService")

local stamina = 100
local running = false

stamina = math.clamp(stamina,0,100)

UserInputService.InputBegan:Connect(function(input)
    if input.KeyCode == Enum.KeyCode.LeftShift then
        running = true
        character.Humanoid.WalkSpeed = 32
        while stamina > 0 and running do
            stamina = stamina - 1
            script.Parent:TweenSize(UDim2.new(stamina / 100,0,1,0), "Out","Linear", 0)
            wait()
            if stamina == 0 then
                character.Humanoid.WalkSpeed = 16
            end
        end
    end
end)

UserInputService.InputEnded:Connect(function(input)
    if input.KeyCode == Enum.KeyCode.LeftShift then
        running = false
        character.Humanoid.WalkSpeed = 16
        while stamina < 100 and not running do
            stamina = stamina + 1
            script.Parent:TweenSize(UDim2.new(stamina / 100,0,1,0), "Out","Linear", 0)
            wait()
            if stamina == 0 then
                character.Humanoid.WalkSpeed = 16
            end
        end
    end
end)