# Tutorial: Script a trigger
BeginnerDesigner
In this tutorial, we'll create a [trigger](triggers.md) that doubles the size of a ball when the ball passes through it.
>[!Note]
>The screenshots and videos in this tutorial were made using an earlier version of Stride, so some parts of the UI, and the default skybox and sphere, might look different from your version.
## 1. Create a bouncing ball
Follow the instructions in the [Create a bouncing ball](create-a-bouncing-ball.md) tutorial. This creates a simple scene in which a ball falls from midair, hits the ground, and bounces.
## 2. Add a trigger
Now we'll add a trigger between the ball and the ground, so the ball passes through it.
1. In the **Scene Editor**, click the white plus button (**Create new entity**) and select **Empty entity**.
Game Studio adds an entity to the scene with the default name **Entity**.
2. This entity will be our trigger, so rename it *Trigger* to make it easy to identify.
3. Since we don't need the trigger to move, we'll make it a [Static Component](static-colliders.md). In the **Property Grid**, click **Add component** and select **Static component**.
4. In the **Property Grid**, expand the **Static Component** to view its properties.
5. We need to give the trigger a shape. Next to **Colliders**, click  (**Add**) and select **Box**.
This gives the trigger a box shape.
## 3. Give the trigger a model
Right now, the trigger is invisible at runtime. To better show how the trigger works, we'll make it a transparent box. This has no effect on how the trigger works; it just means we can easily see where it is at runtime.
1. Create a new procedural model asset. To do this, in the **Asset View**, click **Add asset**, and select **Models > Cube**.
2. Create a new empty material asset. To do this, in the **Asset View**, click **Add asset**, and select **Materials > Material**.
3. Let's rename the material to make it easy to identify. To do this, right-click, select **Rename**, and type a new name (eg *Transparent*).
4. Select the **Trigger** entity. In the **Property Grid**, click **Add component** and select **Model**.
Game Studio adds a model component to the entity.
5. Under **Model**, click  (**Select an asset**).
6. Select the **Cube** model we created in step 1 and click **OK**.
7. In the **Property Grid**, under **Model > Materials**, click  (**Select an asset**).
8. Select the **Transparent** material we created in step 2 and click **OK**.
9. In the **Asset View**, select the **Transparent** material asset.
10. In the **Property Grid**, under **Misc > Transparency**, select **Blend**.
11. By default, the Alpha is set to 1. This makes the material completely opaque. To set it to 50% opacity, set the **Alpha** to 0.5.
Now the trigger area will be visible at runtime.
## 4. Position the trigger
We need to position the trigger between the ground and the sphere, so the ball falls through it.
In the **Property Grid**, under **Transform**, set the **Position** to: *X:0, Y:3, Z:0*
Now the trigger entity is between the ground and the sphere.
## 5. Change the sphere size with script
If we run the project now (**F5**), the ball bounces off the trigger, but nothing happens.
Let's write a script to change the size of the ball when it enters the trigger.
>[!Note]
>For more information about scripts, see [Scripts](../scripts/index.md).
1. In the **Asset View**, click **Add asset** and select **Scripts** > **Sync Script**.
2. In the **Create a script** dialog, name your script *Trigger* and click **Create script**.
2a. If Game Studio asks if you want to save your script, click **Save**.
2b. If Game Studio asks if you want to reload the assemblies, click **Reload**.
3. Open the script, replace its content with the following code, and save the file:
```cs
using Stride.BepuPhysics;
using Stride.BepuPhysics.Definitions.Contacts;
using Stride.Core.Mathematics;
using Stride.Engine;
namespace TransformTrigger
{
// Adding IContactHandler to listen to contact events
public class Trigger : SyncScript, IContactHandler
{
public override void Start()
{
// Initialization of the script.
}
public override void Update()
{
// Do stuff every new frame
}
// Let objects pass through this trigger, false would make objects bounce off it
public bool NoContactResponse => true;
void IContactHandler.OnStartedTouching(Contacts contacts)
{
// When something enters inside this object
contacts.Other.Entity.Transform.Scale = new Vector3(2.0f);
}
void IContactHandler.OnStoppedTouching(Contacts contacts)
{
// When something exits this object
contacts.Other.Entity.Transform.Scale = new Vector3(1.0f);
}
}
}
```
This code doubles the size (scale) of any entity that enters the trigger. When the entity exits the trigger, it is set to a unit size.
4. Reload the assemblies.
## 6. Add the script
Finally, let's add this script to the trigger entity as a component.
1. In **Game Studio**, select the **Trigger** entity.
2. In the **Property Grid**, click **Add component** and select the **Trigger** script.
3. Inside your **Static Component** click on the hand icon next to the `Contact Event Handler` property
4. Select your newly created `Trigger` component and press `Ok`
## 7. Run the project
Run the project (**F5**) to see the trigger in action.
The ball falls through the trigger, doubles in size, exits the trigger, and returns to its normal size.
## See also
* [Tutorial: Create a bouncing ball](create-a-bouncing-ball.md)
* [Colliders](colliders.md)
* [Collider shapes](collider-shapes.md)
* [Scripts](../scripts/index.md)