Have a taste test experiment in which the students examine the tastes of food items with real sugar in them and compare them to items with sugar substitutes. To make this a valid test, choose several different types of food, such as candy, cookies and soda. Place the sugar and sugar-free versions in foam cups labeled A, B and C. That way the kids don't know which is which. The students will rank the items in order according to which ones they like best. In the end, tally the results and discuss them. The point here is to show the students that, to an extent, their preconceptions about flavor may well affect their perception of it.
Foods often don't taste the same when you have a stuffy nose. The reason for this is that your sense of smell is actually involved in a lot of what people think of as their sense of taste. To demonstrate this, create or purchase a delicious-smelling food for the kids to eat. Something with a lot of oils in it works well for this demonstration. Divide it into two portions. Have the eighth-graders eat the first portion just as they would normally. Have them hold their noses shut with one hand while they eat with the other so they can notice the difference.
Have a round of blindfolded taste identification. Bring in a number of items in opaque containers with lids. Number these items and have each child taste each item, remove their blindfolds and write what they think the items are. When the students have gone through all the items, discuss the results. It can be interesting to see how one person's interpretation of a taste differs from another.
Have the kids taste a piece of candy that has very little sugar in it, and have them rinse their mouths with salt water. Have them taste the candy again. They will be surprised to find the candy now tastes much sweeter than before. This is because taste is in part a relative sense, and compared to the salt, the candy tastes much sweeter.