Collision event occurs when two sprites collide.
You could find the involved sprites with Sprite1 and Sprite2 properties.
Example:
Sub gw_Collision
'This event occurs when two sprites collide.
spr1.Value = gw.Sprite1 'Get
the two sprites.
spr2.Value = gw.Sprite2
If gw.SpriteIndex(spr1.Value) = 0 OR gw.SpriteIndex(spr2.Value) = 0 Then Return
'If one of the sprites is
the explode sprite (which is the first
sprite) we won't do anything.
spr1.MarkedForDelete = true 'Delete
these sprites.
spr2.MarkedForDelete = true
explodeSprite.X = (spr1.X + spr2.X)/2 - explodeSprite.Width / 2 'Set the explode sprite position
explodeSprite.Y = (spr1.Y + spr2.Y) /2 - explodeSprite.Height / 2
explodeSprite.CurrentFrame = 0 'Start the exploding animation from
the first frame.
explodeSprite.LifeTimeCounter = 5 'Activate the sprite for 5
ticks.
Sound("explode.wav") 'Plays the sound.
End Sub