One of the immutable properties of gravity is that the acceleration of an object due to gravity is constant, not counting wind resistance. Demonstrate this fact with a game. Break your class into pairs and challenge students to "race" objects to the floor by dropping them simultaneously from the same height. Students will find that the race is always a tie. If students use an especially light object, such as a feather, explain why wind resistance slows it down.
Some substances, such as helium, are lighter than air and therefore seem to defy gravity. Give each student in the class a balloon filled with helium. Allow students to decorate their balloons with colored markers. Have students release their balloons in the classroom at the same time, floating them up to the ceiling. After a couple of days, the balloons should start to descend. The student whose balloon remains on the ceiling the longest wins. Use the game as a means to explain to students why helium makes objects float, and why the slow release of helium makes them succumb to gravity once again.
Using equations to solve gravity-related problems is best left to older students, but there are some basic facts of gravity that you can teach to first-graders. Make a trivia game that tests students' knowledge of these facts. Include such trivia questions as, "True or false: gravity is defined as attraction between objects with mass" and "Which has a higher gravity: Earth or Mars?" Ask the questions out loud and give a point to the first student to raise his hand and answer correctly.
You can teach students about the properties of gravity with an online game. "Gravity," at Primarygames.com, asks the player to move a ship from one place to another using the keyboard. Players must account for gravitational pull, as the ship starts to fall as soon as you cut the engine. With an alien-invasion back story and colorful graphics, this game is sure to hook first-graders as they get the feel for gravitational pull.