I have been waiting nearly two years to share this with you, because I was hoping to find the appropriate lab-ware to borrow so I could photograph this VERY cool demonstration. But, it hasn't happened yet and doesn't look like it will in the near future, so I've decided to share it with you, without any photographs. I hope you'll try it. If you do, maybe you'll take some pictures and be interested in sharing them!
Pyrex has the same index of refraction as corn oil. As such, this demonstration will only work with corn oil, not with any other kind of oil.
Put some CORN oil in a beaker.
Place an empty test tube in the corn oil - it will appear bent due to refraction. Then, begin to fill the test tube with corn oil - the test tube will disappear! This could also be done with a small beaker.
If you have a mature audience, try this trick… place a broken test tube (be VERY careful, Pyrex is SHARP) into the beaker of oil (which already contains the hidden test tube), telling the students that it’s a restorative potion. Then pull out the whole, hidden test tube!