Gravity

Gravity is a force that appears in all Mario, Wario and Yoshi series games which sucks Mario, Wario, Yoshi or Luigi towards the ground and sticks them there, only allowing them to jump.

Yoshi's Universal Gravitation
In Yoshi's Universal Gravitation, gravity makes it's first notable appearance. However, it only appears in a few levels. But if the player tilts the GBA left or right, the world will tilt accordingly, which doesn't appear in any other Mario or Yoshi game.

Super Mario Galaxy
Gravity appears in all games but it makes its biggest appearance in the game Super Mario Galaxy where, in multiple levels, there are many different forms of gravity. Black Holes use gravity to suck Mario or Luigi into the black hole and kill them when they accidentally leave the surface of a planet with a very weak gravitational pull. Some of the other Planets have a stronger pull and can pull Mario back to them no matter how far away he goes from them unless he uses a Pull, Sling or Launch star. Gravity also appears as a force to be manipulated in several levels where gravity pulls Mario towards special walls called Gravity Walls. There are also two other manipulations of gravity in that game, first off Gravity Arrows which will change the flow of gravity in certain areas and Gravity Spotlights which either increase or decrease gravity; sucking Mario towards another place or allowing him to stick to other surfaces.

Super Mario Galaxy 2
Again, gravity appears in Super Mario Galaxy 2, having the same purpose as in Super Mario Galaxy.