How do I add a new record ith belongsToMany and hasMany relationships using a pivot table?

I just setup model Asset with belongsToMany rooms and model Room with hasMany assets relationships. I also created a pivot table that stores both the ids of room and assets.

MODELS

class Asset extends Model
{

    protected $fillable = [ 'name', 'slug', 'price' ];

    public function room()
    {
        return $this->belongsToMany('App\Room');
    }
}

class Room extends Model {

    protected $fillable = [ 'number', 'status' ];

    public function assets()        
    { 
        $this->hasMany('App\Asset'); }
    }
}

MIGRATIONS

public function up()
    {
        Schema::create('assets', function (Blueprint $table)
        {
            $table->increments('id');
            $table->string('name')->unique();
            $table->string('slug');
            $table->string('price');

            $table->timestamps();
            $table->softDeletes();
        });
    }

 public function up()
    {
        Schema::create('asset_room', function(Blueprint $table) {
            $table->integer('asset_id')->unsigned();
            $table->foreign('asset_id')->references('id')->on('assets');
            $table->integer('room_id')->unsigned();
            $table->foreign('room_id')->references('id')->on('rooms');
            $table->unique(['asset_id', 'room_id']);
            $table->timestamps();
        });
    }

I added an asset via php artisan tinker as:

$asset = new App\Asset;
$asset->name = "Lamp";
$asset->slug = "lamp";
$asset->price = 40;
$asset->save();

Now how do I add an asset to a room so that it also adds an entry to the pivot table?

$room = new App\Room;
$room->number = 1;
$room->status = 1;
$room->asset...?
$room->save();

via Chebli Mohamed