Opening Overwatch loot boxes or Halo 5 REQ packs adds a special drama to a gaming session. The crate shakes. A jingle chimes. Lights peek out from the cracks. It swells with potential. Game developers make subtle design decisions that stoke the hope that keeps players opening mystery boxes, crates and packs. And not just on the stats side of things—just as important are the cosmetics of the experience.

A combination of visual and aural factors can make it feel like Christmas, and that's intentional according to people we recently spoke to who explained the designs of three totally different mystery mechanics.

Here's the thinking behind each:

Overwatch Loot Boxes

During Overwatch's Lunar New Year event, I tore through loot boxes with a single-minded goal: Roadhog's "Bajie" skin. It drove me crazy. I'd watch the special edition box's gears churn and shake until it exploded in fireworks. When the items hit the ground, gold nuggets rained from the sky. The whole experience is kinetic, a song and dance of lights and movement. It dulled the pain of never getting that skin.

Overwatch loot boxes are like live animals, active and full of mystery. From the plain loot crate to the Christmas gift to the Lunar New Year fireworks, they always feel like wrapped presents, hiding something that could satisfy some desire. That's because their developers Michael Heiberg and Jeremy Craig were deliberate in their designs. Originally, for their Halloween crate, they briefly considered a sinister jack-o-lantern before deciding it didn't give off that "present" vibe. "It looked really cool and hit the Halloween theme well, but it didn't evoke a feeling of being inviting and desirable," Heiberg said. But he acknowledged that most people don't really want to open up jack-o-lanterns. "Instead, we went with more of a jack-o-lantern themed candy bucket that (if you have a sweet tooth like me) definitely hits those notes."

Overwatch box animations maximize on anticipation. They break open, shake, spit into the air and rain down items, never revealing what you receive until the very end. The rewards almost feel tacked-on to the opening experience. "When you start opening a loot box, we want to build anticipation," Heiberg said. "We do this in a lot of ways — animations, camera work, spinning plates, and sounds. We even build a little anticipation with the glow that emits from a loot box's cracks before you open it." Originally, colored lights preceding the spinning plates hinted at the items' rarity. It drew the eye to one item in particular at the expense of others. "We quickly learned that this was too early, and it killed your anticipation of the box's contents," Heiberg said.

Duelyst Mystery Crates

Collectible card game Duelyst's mystery crates look like some combination of layer cake and wedding ring. They come in tiers: common, rare and epic, each garnished with the requisite amount of gold. "We wanted them to look like nice, futuristic Pandora's boxes," Counterplay Games co-founder Emil Anticevic told me. The "Intricate metalwork, hyper-stylized" look was originally drawn by hand, and then rendered in 3D. The sound design, which Anticevic describes as "a lightly-shaking silver spoon," adds to their tinkery, jewel-like feel. "There's a physicality to they key and the crate opening, but once the items are revealed one by one, you hear a subtle chime that feels close, but not assertive," Anticevic explained.

Unlike games like FIFA 17, Duelyst wanted a quick opening experience. There's no reveal animation. The anticipation mostly rests in the crates' design, which Anticevic says he crafted to look valuable. "The crate blows open, each side flies off, and we shoot off some wisps of light that carry into the cosmetics [items]," Anticevic said. He emphasized that the design isn't aimed at players who want to open several crates—they steered clear of "gacha" design decisions.

Halo 5 REQ Packs

YouTubers are obsessed with opening Halo 5 REQ packs in long, rushed strings, sometimes over a hundred at a time. That's because they feel a little like potato chips—perfectly understated and best consumed in bulk. The packs are plain, Spartan card designs that, when opened, burst into about a dozen other cards displaying won items. Confetti and a small tinkling sound or cheer accompany their opening, but essentially, the design is totally downplayed, even for higher-tier packs.

"We never give anybody everything they want right away," Halo 5 lead progression designer Christopher Bloom told me. "We designed from a place of empathy. We wanted it to be this festive Christmas experience. You're waiting for your kid to open a package, but you can't do it all the time or you lose the specialness."

Bloom didn't want to the REQ packs to be too ornate or the opening experience to be too drawn-out, or it could take away from the rush of opening several at once. His team didn't want to "distract the player too much," he said. "You don't want to make anything too long because some users open multiple in a row. You don't want to frustrate them too much."

Bloom explained that the packs' pulsating opening can feel a little like slot machines or a dealt poker hand. "It's the possibility you might get extremely lucky. Ideally there's value in everything you open. You like what you get but there's always something else you kind of want to get."

What are your favorite mystery mechanics and why? Tell us in the comments.