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The Laser That Actually Kills

A killbrick is four lines, and three of them exist to answer the question Touched refuses to: whose leg was that. Trace one touch from the limb it hit all the way back to the player it belonged to.

Read first: Reaching Into the Tree From Code

A killbrick is four lines, and three of them exist to answer the question Touched refuses to: whose leg was that. Trace one touch from the limb it hit all the way back to the player it belonged to.

What Touched actually hands you

You know the shape already. What matters here is being precise about the argument, because everything else in the chapter follows from it.

Luau

local laser = script.Parent laser.Touched:Connect(function(otherPart)    print(otherPart.Name)         --> LeftLowerLeg    print(otherPart.ClassName)    --> MeshPart    print(otherPart.Parent.Name)  --> Amaraend)

A leg. Not a player, not a character, not something with health. The player's name is one level up, and the thing that can actually be damaged is up there too — as a sibling of the leg, not as one of its children.

A character model is flat: every limb, the Humanoid, and the HumanoidRootPart all sit as direct children of the model. That flatness is what makes one step up exactly the right distance, and it is also what breaks when a hat is involved.

Climbing from a limb to a Humanoid

The Humanoid is the object that makes a model a character. It holds Health, controls walking and jumping, and is what dies. Damage is something you do to a Humanoid, never to a part.

Luau

laser.Touched:Connect(function(otherPart)    local humanoid = otherPart.Parent.Humanoid   -- works, and is fragile    humanoid.Health = 0end)

This is the version in most tutorials and it is one falling brick away from breaking the laser. A brick's parent is the Workspace, the Workspace has no child called Humanoid, and a dot lookup for a child that is not there throws immediately: Humanoid is not a valid member of Workspace. An unhandled error inside a Touched handler kills that invocation — so the Output window fills with red every time debris lands on the laser, and the real problem, a lookup with no guard, is easy to mistake for something mysterious.

FindFirstChildWhichIsA is the safe climb

Two changes make it correct. Search by class rather than by name, and check the result before using it.

Luau

local laser = script.Parent laser.Touched:Connect(function(otherPart)    local humanoid = otherPart.Parent:FindFirstChildWhichIsA("Humanoid")    if not humanoid then return end     humanoid:TakeDamage(100)end)

Searching by class rather than name matters because nothing guarantees the Humanoid is called "Humanoid". It usually is, and a custom character rig or a badly behaved free model can rename it, at which point a name lookup fails and a class lookup does not.

The if not humanoid then return end is the whole safety of the script. It is the guard from the chapter on nil, in the place it matters most.

From the part that touched to the player who owns it

1otherPart→MeshPart "LeftLowerLeg"
2otherPart.Parent→Model "Amara"
3:FindFirstChildWhichIsA("Humanoid")→Humanoid
4humanoid:TakeDamage(100)→Health 100 → 0

The player dies. This is the case the killbrick was written for.

Every limb of a character is a direct child of the character model, and the Humanoid is a sibling of those limbs. One step up is exactly the right distance.

The hat case is worth dwelling on, because it is a real bug that ships in real experiences. A hat's Handle lives inside an Accessory, which lives inside the character — so otherPart.Parent is the Accessory, and the Humanoid is the Accessory's sibling rather than its child. The search finds nothing and the player walks through your laser wearing a top hat.

Do not fix it with a second .Parent. That would break the ordinary leg case, which is far more common. Search upward instead, and both work.

Luau

laser.Touched:Connect(function(otherPart)    local character = otherPart:FindFirstAncestorOfClass("Model")    if not character then return end     local humanoid = character:FindFirstChildWhichIsA("Humanoid")    if not humanoid then return end     humanoid:TakeDamage(100)end)

TakeDamage and Health are different choices

Both will kill a player and they are not interchangeable.

humanoid:TakeDamage(n)

Subtracts n, and respects a ForceField. A player who has just spawned is protected, which is what stops a laser next to the spawn point killing people before they can move.

humanoid.Health = 0

Sets health directly and ignores every protection. Correct for a void floor where nothing should survive; wrong as a default, because it makes spawn protection meaningless.

Prefer TakeDamage unless you have specifically decided that nothing should save the player. Passing 100 kills a default character outright, and passing 25 gives you a laser that takes four hits — which is a design decision you now get to make.

Getting from a character to a Player

A character is a model in the Workspace. A Player is the account behind it, living in the Players service, and holding everything that should survive a death — leaderstats, checkpoints, settings.

Luau

local Players = game:GetService("Players")local laser = script.Parent laser.Touched:Connect(function(otherPart)    local character = otherPart:FindFirstAncestorOfClass("Model")    if not character then return end     local player = Players:GetPlayerFromCharacter(character)    if not player then return end   -- an NPC has a Humanoid and no Player     local humanoid = character:FindFirstChildWhichIsA("Humanoid")    if humanoid then        humanoid:TakeDamage(humanoid.MaxHealth)        print(player.Name, "hit the laser")    endend)

GetPlayerFromCharacter returns nil for anything that is not a real player's character, which makes it a precise filter as well as a lookup. An NPC with a Humanoid passes every earlier check and fails this one — so if you want the laser to kill NPCs too, this is the line you leave out on purpose.

Key takeaways

Touched hands you a part. The player's name is one level up, and the damageable object is a sibling of that part.
A character model is flat — every limb, the Humanoid, and the HumanoidRootPart are direct children of it.
otherPart.Parent.Humanoid throws 'Humanoid is not a valid member of Workspace' the moment a falling brick touches the laser.
FindFirstChildWhichIsA("Humanoid") searches by class, so it survives a rig whose Humanoid was renamed.
if not humanoid then return end is the whole safety of the script. Without it, one brick produces a stack trace.
A hat's Handle is one level deeper than a limb. FindFirstAncestorOfClass("Model") handles both; a second .Parent breaks the common case.
TakeDamage respects ForceFields and spawn protection; setting Health = 0 ignores everything. Prefer TakeDamage.
Use TakeDamage(humanoid.MaxHealth) rather than a literal 100, in case MaxHealth is not the default.
GetPlayerFromCharacter returns nil for NPCs, which makes it a filter as well as a lookup.

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