Build a mini transport system

What can I use if I don't have a bottle cap, toy vehicle, pencil, or wooden skewer?

Use a small folded cardboard platform or a LEGO plate taped to a tiny block instead of the bottle cap or toy vehicle, and substitute a chopstick, thin dowel, or straw for the pencil/skewer when you lay the axle across the towers (steps: make a cargo carrier; lay a pencil or wooden skewer across the top of the two towers).

Why does the cargo stop or tip on the ramp and how can we fix it?

If the cargo stops or tips, add extra tape where the ramp meets the station to steady the slope, rebalance the carrier by taping its base flat to the toy vehicle or platform, and then adjust the ramp height or add the rubber band brake until it moves at a steady speed (steps: tape each ramp to a station; make a cargo carrier; adjust the ramp height or add a rubber band brake).

How can I adapt the activity for different age groups?

For ages 3–5 pre-cut the ramps and let them place stations and move the carrier, ages 6–8 can cut and tape the ramps and towers and thread the string, and ages 9+ can add a second pulley, vary cargo weights, and measure run times to study friction (steps: pick where stations will sit; cut two ramp pieces; build two cardboard towers; loop a piece of string over the pencil).

How can we extend or personalize the mini transport system to make it more challenging or creative?

Decorate the stations, add extra towers for a multi-stage pulley, test different cargo weights like bottle caps versus small boxes, attach a ruler and stopwatch to record times, and then share your modified design and your one-sentence conclusion about gravity and friction on DIY.org (steps: build towers; tie string to the cargo carrier; place the cargo on the ramp; draw or write one sentence).