During your time at Black Mesa East (Chapter 5), before the Combine attack, you can engage in a brief, optional activity with D0g. Alyx will encourage you to play fetch with him. You can use the Gravity Gun to throw objects for D0g to retrieve. This is a lighthearted moment that showcases the game's physics engine and D0g's personality. While it offers no tangible rewards, it's a charming interaction that adds to the character development and world-building.
Half-Life 2
D0g's Playtime (Black Mesa East)
Engage in a charming moment with D0g in Half-Life 2's Black Mesa East. Use the Gravity Gun to play fetch and enjoy this unique interaction.