From now on this is how I am going to start each semester teaching fluid mechanics. The definition of a fluid that is use is

*A fluid is a material that deforms continuously under the action of a shear stress.*

This demonstration illustrates the continuous deformation.

**Equipment**

- Overhead projector
- Two transparency sheets
- A viscous liquid such as dish soap (easy to clean up) or honey (not so easy to clean up)
- A text book (they have to be good for something)

**Demonstration**

- Remind the class that a when you apply a shear force to a solid it deflects a finite amount
- Demonstrate this by applying a shear to the text book. The book is a useful prop because you can get it to noticeably deflect.
- Place one of the transparencies on the OHP and pour some of the soap onto the middle of the sheet
- Place the second sheet on top of the first.
- Slowly pull the two sheets apart horizontally while stating that a fluid differs from a solid in that a fluid deforms continuously under the action of a shear stress.

I then write the definition on the board and launch into fluid properties. Obviously there is no analysis to be done 5 minutes into the first class of a fluids course, but it can help students clearly visualize the difference right from the start.

An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

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