Students during the course of this activity are exploring the relationships between the natural and constructed worlds. Place a piece of tape on the surface of the sphere. The students can change their positions by moving along the circumference of the balloon to discover when they can no longer see the marker. Ask them to place one eye very close to the surface of the sphere and look towards the edge. The edge that they see is like the Earth's horizon.
Weather balloons are available from science supply centres.
Students compare the disappearance of the object on the surface of the sphere as they move to the appearance or disappearance of grain elevators as one moves toward or away from them. Get samples of soil from four or five different locations, ensuring that the soils are quite distinct. Examine the soil samples with hand magnifiers. Each student group could be responsible for examining and describing one sample and explaining the findings to the rest of the class, or each group could have a chance to compare all the samples. Half fill a jar with water, add about one-quarter of a jar of a soil sample, cap the bottle, and shake.