# Collision detection: do two objects touch?

[Concepts explained](README.md)

**Collision detection** asks whether two shapes touch or overlap. A game can use the answer to award a coin, stop movement, or lose a life. Detecting contact and deciding what happens afterward are separate jobs.

## Compare two number-line segments

Here the shapes are simple segments on one horizontal line:

```pliro
# language: en
let firstLeft = 10
let firstRight = 30
let secondLeft = 25
let secondRight = 40
say firstLeft <= secondRight and secondLeft <= firstRight
```

The output is `true`. The segments share positions from 25 through 30. With these comparisons, touching at an endpoint also counts. Each segment must have its left endpoint at or before its right endpoint.

A two-dimensional rectangle test needs to check both horizontal and vertical ranges. A three-dimensional test has another direction to consider.

## The shape used for contact

A **hitbox** is a simple shape chosen to represent an object for collision checks. It may differ from the visible picture. That can make checks easier, but a large hitbox may feel unfair to a player.

Pliro's `touching3D` tests contact between supported 3D objects. It does not automatically move them apart or act as a 2D sprite collision function. Read its [reference](../builtins/touching3D.md) for the actual geometry rules.

Try setting `secondLeft` to 31 in the example. Predict why the answer changes.