Here's why:
* Move Tutors: The game doesn't have move tutors that can teach Explosion.
* TMs: Explosion is not a TM in Pokémon XD.
* Breeding: While you can breed Pokémon to inherit moves, Explosion isn't a move that can be passed down through breeding.
However, there are other powerful moves you can teach Glalie:
* Ice Shard: A fast and powerful Ice-type move that can take down opponents quickly.
* Earthquake: A powerful Ground-type move that can hit many Pokémon hard.
* Blizzard: A powerful Ice-type move with a chance to freeze opponents.
You can teach Glalie these moves with the help of move tutors, TMs, or breeding.