Icicle

Icicles are frozen water drops that have sometimes appeared in the Mario franchise.

Mario Bros.
Icicles first appear during Phase 16 (17 in Japan) of Mario Bros. as obstacles that drop from under the top platforms and pipes, where they would constantly regenerate. They kill the player if they fall upon them; however, icicles can also be deadly if touched while they are still attached to the ceiling. Touching an icicle while it is still a liquid droplet or using the POW Block will prevent it from being formed, though this will not award any points. However, the POW Block will also cause fully formed icicles to drop prematurely.

Icicles return in Luigi Bros., a slightly modified version of Mario Bros. featured in Super Mario 3D World. Here, they act the same way as previously.

Super Mario Bros. Special
In Super Mario Bros. Special, icicles could be defeated by fireballs. They do not regenerate.

Super Mario World 2: Yoshi's Island
In Super Mario World 2: Yoshi's Island and Yoshi's Island: Super Mario Advance 3, throwing an egg at them would cause them to fall down from the ceiling.

Super Princess Peach
Icicles reappeared in Super Princess Peach in the Gleam Glacier world and behaved the same way as they did in Mario Bros..

Wario Land: Shake It!
Icicles appear in Wario Land: Shake It!, in the level Slipshod Slopes. They appear in a cove part, and seem to be gigantic. When executing the Earthshake Punch, these big icicles will fall down, and crush any enemy underneath (defeating 5 enemies with those icicles is also a mission in that level). When Wario hits a falling icicle, he'll get hurt. Wario can also stand on those icicles as if they are platforms. However, the fallen icicles will break after a short time. when broken, new icicles will grow on the piece of ceiling the old ones were fallen off. There are giant icicles as well in Freezing Fields.

Super Smash Bros. Brawl
Icicles also make a minor reappearance in Super Smash Bros. Brawl. When fighting on the Ice Climbers' stage, the Summit, they act the exact same way they did in the original Mario Bros., except this time they are indestructible and cannot be melted.

Super Smash Bros. for Wii U
Huge Icicles are featured as stage hazards on the Mushroom Kingdom U stage in Super Smash Bros. for Wii U with their appearance from the New Super Mario Bros. series. They are present in all four variations of the stage. They occasionally fall from the top of the screen and can damage fighters.

New Super Mario Bros. Wii
Icicles are found in World 3-3 and 3-Castle of New Super Mario Bros. Wii, along with giant indestructible icicles known as Huge Icicles that Mario can walk on after they fall, some of which may tilt to one side. Icicles come long and short, and when the icicles fall similar to a Thwomp, they reform after a few seconds. Mario and company, as well as enemies, will take damage if they come in contact with one. There are also some that don't even fall at all but still hurt the player. Icicles can be defeated with a Super Star or by hitting a block under it as it is falling.

New Super Mario Bros. U
They also appear in New Super Mario Bros. U and New Super Luigi U. In both games, they can be found in Frosted Glacier-Tower, Frosted Glacier-5, and Frosted Glacier-Castle. Inside Wendy's airship from the latter level, she uses magic to cause icicles to fall onto Mario, Luigi, and the two Toads. They can also be found in World Coin-7. In Boost Mode, tapping on an icicle in its idle state will cause it to break and never reform, although tapping a falling one will break it but will still eventually reform. Additionally, destroying one of Wendy's icicles with Boost Mode will destroy it but will then immediately reform.

Donkey Kong Country: Tropical Freeze
In addition, icicles can be encountered in Donkey Kong Country: Tropical Freeze during Icicle Arsenal, a level on Donkey Kong Island. They are remarkably sharp and long, slightly varying in size. They hang from ceilings and drop down when the Kongs walk underneath them. Moments before falling, they signal the player with a slight shake. The Kongs lose one heart from their health meter if they get crushed by an icicle; similarly, an enemy gets defeated if hit by one such obstacle. After coming down on either the ground or a character, an icicle breaks and yields a single banana.