**Discussion:** An understanding of the nature of fundamental particles helps students recognize both the complexity and simplicity of nature. Just as all the words in the English language are combinations of subsets of 26 letters, atomic physics showed that atoms of the many elements are combinations of three particles - the proton, neutron and electron. As the number of "elementary particles" identified in cosmic ray showers and other high energy interactions proliferated, some began to believe they were complex, composite particles created from a few, more fundamental particles. This activity will help students identify common elements of their "atoms" and also suggest that what is determined to be fundamental indeed has a substructure - an introduction to the Standard Model.

**Activity:** Using standard size index cards, place them in multiples of three into a number of standard size envelopes. For example, for a class size of 25 students, prepare 250 envelopes as follows.

- 42 envelopes of 3 index cards per envelope
- 52 envelopes of 6 index cards per envelope
- 42 envelopes of 9 index cards per envelope
- 40 envelopes of 12 index cards per envelope
- 30 envelopes of 15 index cards per envelope
- 28 envelopes of 18 index cards per envelope
- 16 envelopes of 21 index cards per envelope

Mix up the prepared envelopes and distribute 10 to each student. Each envelope may be used to represent a "particle" as identified in some high energy reaction. Have the students mass their "particles" and share this information on a class data chart, or enter into a computer graphing program. (A student worksheet is available if you are not using a computer graphing program.) The use of top loading analytical balances, with rounding to the nearest 0.1g will greatly speed the data collection process.

Examine the graph and note similarities in masses. Group and regraph the data; note that all differences occur in some "fundamental" unit, i.e. the mass of three index cards. After students have predicted the mass of the "fundamental particle" discuss how you might investigate if this fundamental particle itself might have an internal structure. Suggest that if greater energies were used to look inside the particle one might discover its structure. When an envelope is opened students will notice that what they had assumed to be the "fundamental" unit was in fact itself made of three smaller particles just as the proton and neutron each have an internal structure made of three quarks. Sample graphical results have been included for your use in discussing the results.

This activity takes about one 55-minute period to complete.