Asteroids

TThing = class
  procedure CollideWith(AOther: TThing); // default: elastic collision
end;
TSpaceShip = class(TThing)
  procedure CollideWith(AAsteroid: TAsteroid); // bang!
end;
TAsteroid = class(TThing)
  procedure CollideWith(ASpaceShip: TSpaceShip); // bang!
  procedure CollideWith(ABullet: TBullet); // destroy Asteroid
end;
TBullet = class(TThing)
  procedure CollideWith(AAsteroid: TAsteroid); // destroy Asteroid
end;

for thing in allThings do
begin
  thing.move;
  for otherThing in allThings do
    if atSamePlace(thing, otherThing);
      thing.collideWith(otherThing);
end;