Technically, since sunlight is energetic, it imparts a force against whatever it hits. The force of sunlight is approximately 4 pounds per square mile.
So, you could argue that a shadow has negative weight.
http://io9.com/5966470/this-is-the-weight-of-sunlight
Science!
Science!
Those of us of a certain age will get this...
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[unless it's too obscure, and I'm being a weirdo...which is possible...]
