Sink or Float Bottle  Place items that are heavy enough to sink in water, like marbles, in an empty plastic bottle.  Place items in the bottle that will float, like straws that have been cut into little pieces.  Fill the bottle with water. Seal shut with hot glue.  Let children perdict what will happen and then shake the bottle vertically, horizontally and upside-down. Children should be able to see which items float to the surface and which items sink.
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