After building the dice model and looking at the more complicated
Nuclear
decay model, students observe this model and the attached graphs. This
model was built to show students that there can be a series of decays and
that they can occur at various times.
The stocks are animated in the classroom version, and this adds to
student understanding. After viewing it the first time, students wanted
to adjust the graph so that there was only one scale on the left. They
quickly discovered that this "hides" much of what is going on because of
relative size. It is presented here with the multiple scale format.
Vensim Version 
STELLA Version

[Diagram Level  Equations
Level Graphs ]
Amt_Isotope_1(t) = Amt_Isotope_1(t  dt) + ( Decay_1) * dt
INIT Amt_Isotope_1 = Init_Amt_Isotope_1
Decay_1 = LOGN(2)*Amt_Isotope_1/Halflife_1
Amt_Isotope_2(t) = Amt_Isotope_2(t  dt) + (Decay_1  Decay_2) * dt
INIT Amt_Isotope_2 = 0
Decay_2 = LOGN(2)*Amt_Isotope_2/Halflife_2
Amt_Isotope_3(t) = Amt_Isotope_3(t  dt) + (Decay_2  Decay_3) * dt
INIT Amt_Isotope_3 = 0
Decay_3 = LOGN(2)*Amt_Isotope_3/Halflife_3
Amt_Isotope_4(t) = Amt_Isotope_4(t  dt) + (Decay_3) * dt
INIT Amt_Isotope_4 = 0
Halflife_1 = 50
Halflife_2 = 0.5
Halflife_3 = 2
Init_Amt_Isotope_1 = 1000
Total_Radioactive = Amt_Isotope_1+Amt_Isotope_2+Amt_Isotope_3
Time Specs
Range: 0  100, dT = 1, Integration Method = RungeKutta 4
