How to check if a player touch lava ?

Roblox humanoids actually have a property for this called "Floor Material" http://wiki.roblox.com/index.php?title=API:Class/Humanoid/FloorMaterial you can read up on it there!

Heres some basic code.

while wait(0.1) do
    if plr.Character.Humanoid.Health ~= 0 then -- Check if they are not dead!
        if plr.Character.FloorMaterial == Enum.Materials.CrackedLava then -- Tell if they are touching lava
            plr.Character:BreakJoints() -- Kill them.
        end
    end
end

My assumption is that the lava terrain is made out of a different material, so incorperating "Color3 GetMaterialColor ( Material material ) Returns current terrain material color for specified terrain material." would probably be the best way.

Not certified