Sink or Float
Start by gathering an assortment of different items, such as a rock, feather, piece of paper, cereal, different toys, Styrofoam pieces, a couple of blocks in assorted shapes, and so on.  Once you have the items gathered fill up your sensory table or some other large tub with water at least 6 inches deep.

Sit down with the kids in group time, and show them each item, asking them if they think it will sink or float.  Record what the consensus is for each item. Then during centers time, have them test their hypothesis.  This part works best as a teacher directed activity; you can do a count down with kids as you drop each item into the water.  Once you’ve done them all, leave the kids to experiment on their own in free play.

Later on in group time, sit down with the kids again and discuss what happened, inviting their ideas about why certain things sank and others floated.  Try to encourage discussions, not just about weight, but size, shape, and density (how heavy an object is for its size).  It’s an activity that introduces kids to many science concepts at once!