E1: When you open the simulation, the double pendulum will start its oscillation. Stop it after some time with Reset, which leaves the old trace and resets the original initial condition. Start again, and observe that traces coincide (deterministic chaos).

Reset after a few oscillations, draw the blue bob very slightly and try to put it into the vertical position again. Can you do that exactly enough to reproduce the old trace for more than a few oscillations?

E2: Increase Speed, which increases the time between calculated points. This will increase the maximum trace length, as the number of calculated points shown is constant. Use Clear when traces become too complex.

E3: Choose Reset and change the length of the secondary pendulum with slider L2/L1. What happens?

E4: Set L2/L1 ~ 0 after Reset. Now you simulate a simple pendulum. Investigate when its oscillation is approximately linear. Is it always periodical? What does the phase space diagram tell you?

E5: As E4; try to pull the bob as exactly as possible into its upper position. Can you stabilize it there? Study repeated oscillations from the highest starting position. Can you reproduce them?

E6: As E5; position the bob near to its stable point (y ~0). Compare the phase space diagram to that of E5.

E7: Change the mass relation of the bobs via Slider m2/m1:  Do that for different length relations L2/L2.

E8:  Add periodic drive (A > 0).

E9:  Change the drive frequency with slider delta

E10:  You can change parameters while the simulation is running.

E11:  After Reset and Clear let the pendulum perform a few oscillations;  then choose Reset without Clear, which leaves the old trace. Pull the blue bob very slightly from its position and restart. Observe the longterm dependence of the trace from slight variations of the starting condition (a1).

E12: Consider how you can realize a double pendulum with periodic drive! One suggestion: 

clamp the end of the primary pendulum to the axis of a small DC motor that is driven by the amplified signal of a sine wave generator.