Now we combine into a model the actions of birth and movement. Begin with a collection of pennies and a styrofoam cup having black and white beads. The fraction of white beads is the probability that a colony divides.

• Begin with a coin on the board at square 0. This indicates one colony at this location.
• Flip each coin on the board. If the coin lands heads, move the coin up one square. If the coin lands tails, move down one square.
• For each coin on the board, pour a bead from the styrofoam cup. If the bead is white, place a new marker on the square with the marker. This indicates that a colony has divided.
• Return to the second step and continue until a marker reaches either square +5 or square -5.
• If the probability of moving upstream and downstream is different from 1/2, then we will need a second styrofoam cup with bead. We can go to three colors of beads to include the possibility that the colony does not move.
• Colony migration depends on the season, so we may want to have a different set of styrofoam cups for different times of the year.