DANGER! 
Squash a Penny on a Train Track
Leverage the force of a locomotive.
1. Pick a location. Find a length of track that is very straight — you want to be able to see and hear the train coming from a long way away. The best location is next to an automated crossing gate — the bells will warn you when a train is coming.
2. Pick a moment. Check the schedule for a gap of at least 15 minutes between trains. Not all train traffic is scheduled, so you must still wait for a time when you can neither see nor hear any trains or crossing bells.
3. Place the penny. Tape the penny to the top of the rail, to prevent the vibration of the approaching train from shaking the penny off the rail. If there’s a bright, shiny part of the rail, tape the penny there; that’s where the train makes the best contact with the rail.
4. Stand back and wait. Stand at least 30 feet away from all tracks, and wait for a train to pass. If the tape doesn’t hold the penny in place, it may come flying out at high velocity.
5. Find the penny. After the train passes, and you can neither see nor hear any trains or crossing bells, find the squashed penny. Be careful, it may still be hot from being squashed. Get away from the track as soon as you have your penny.
Using the tape, it may be possible to get two different types of coins to squash together. Preparing them by sanding their surfaces will increase the chances of a good metallurgical bond. To ensure that you don’t harm the train or the track, never put anything larger than a coin or two on the tracks. 
Gever Tulley
WARNING: Because of the unfamiliar size of train engines, our brains don’t accurately judge their distance and speed. If you see or hear a train, assume that it’s a danger and move to a safe distance immediately. Also, coins may squirt out from under train wheels, so stand at least 30 feet away from where you placed your penny.
Excerpted from Fifty Dangerous Things (You Should Let Your Children Do) by Gever Tulley with Julie Spiegler (fiftydangerousthings.com). Gever is co-founder of Brightworks, a new K–12 school in San Francisco (sfbrightworks.org).