The Earth's layers can be a colorful decoration for your classroom door. Use butcher paper to create a half-circle for each layer. You can also create a cross section of Earth's layers in a full circle, and decorate Earth's surface with oceans and continents. This can create a more accurate illustration for the students. Make sure to clearly label each layer.
Use your door to create a diagram of the water cycle. Use cotton to create realistic clouds, and butcher paper to cut out a lake, raindrops and arrows to show how the water cycle works. You can include animals and humans in the water cycle if you want it to be more detailed. Make sure to include labels for each part of the cycle. You can use the diagram to explain the water cycle before putting it on your classroom door so your students will recognize it.
When teaching students about trees and plants, understanding the functions of their parts will help students remember the individual parts. For example, if students learn that the function of a tree's roots is to absorb nutrients and keep the tree from falling over, they'll remember that roots are under the ground. Create a large tree on your classroom door with butcher paper and label the parts and their functions for the students.
During a human anatomy unit, decorate your classroom door with a diagram of a human skeleton. You may not be able to label all the bones in the diagram, but having the skeleton on the door can still help remind students of the bones and how they're connected in the body. A human skeleton can also make a good seasonal decoration for a science classroom around Halloween.