1996
Bucket of Light
The Bucket of Light is a demonstration of total internal reflection, which I first saw in Roy Glauber's Harvard course "Light and Matter."  
The room is dark.  A laser shines through a hole near the bottom of a two liter soda bottle full of water.  
Removing the cork in the mouth of the bottle releases the vacuum holding the water, and a stream issues forth from the hole.  
A white compound bucket is conveniently at hand to catch the water.  Catching the water in the dark is easy, because all the laser light is trapped in the stream.  Total internal reflection redirects the light into the bucket, which seems to glow, as if the light itself is caught.
The trick is when the water runs out.  A light, the same color as the laser, is in the bottom of the bucket.  Water turns the light on.  It is not obvious while the laser light is following the flow of water into the bucket.  The bucket holder walks off stage with the bucket still glowing.  We have captured a bucket of light.
The demonstration has become easier to do since the advent of inexpensive, powerful laser pointers.   This is the version we put together for Howtoons.  
Liquid Nitrogen Balloon Blow-up
The venue for the Ig's is Sanders Theater.
Sanders Theater is part of Memorial Hall, built to commemorate Harvard's fallen in the Civil War.
Open flames are absolutely forbidden.  No hot fast chemical reactions, which limits the number of possible chemistry experiments.  Liquid nitrogen has become a favorite stand-in for 'real' chemistry.
An ice bucket is prepared by gluing a 20 oz. plastic soda bottle to the inside bottom, using flexible room-temperature-vulcanizing (RTV) silicone.
Liquid nitrogen half fills the bottle.  I place a balloon over the neck of the bottle, and hold the base of the balloon until the round part has inflated a little.  This keeps the balloon from popping off of the neck.
The balloon inflates slowly.  The performing scientists are impatient.  Chins are rubbed, lips are pursed, brows furrowed.  Aha!  We can add hot water to the bucket.  All around the bottle of liquid nitrogen, the hot water accelerates the boil-off.  The balloon rapidly inflates.  It becomes unbelievably huge, before popping with an awesome "Bang!"  
The cold nitrogen gas in the balloon seems to allow the extraordinary inflation.  The sound is louder than a normal balloon popping.  The remains of the balloon appear as if shredded, unlike normal balloon fragments.
This Moment introduced the use of liquid nitrogen (LN2) in Sanders Theater.  We have used LN2 several times since, always to great effect.