Meet the Visionaries Constructing Roller Coasters - In Their Own Backyards

Paul Gregg, a retired aerospace engineer, built the most amazing gift for his grandkids.

A retired Boeing aerospace engineer might be the most popular grandpa in town after he built a roller coaster for his grandkids in his backyard.

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Paul Gregg, who lives outside Seattle, built the coasters in his yard so his grandkids had something to do when they came to his home.

He built the coasters with safety in mind, telling Inside Edition: "Having me or their fathers push the up the lift hill guarantees there is adult supervision. The kids can’t use it by themselves."

In Milwaukee, teenager friends Aidan Deaven and JT Nejedlo built their own roller coaster. They learned how to do it by watching YouTube videos.

"No one officially inspected it hold the weight," Nejedlo told Inside Edition. "We just kind of go for it and hope it works out."

The cart runs on skateboard wheels and they push it up to the top of their launch pad and let it go for a ride.

The cart clocks in at 15 miles per hour and so far, there have been no injuries, a relief to their parents.

In New Jersey, 18-year-old Jadon Brooks built an impressive roller coaster for his little sister, Eden, in their parents' backyard.

Jadon's cart is powered up the hill by cable and sends her off.

Amusement ride safety inspector Walter Reiss checked out Jadon’s coaster and said: "I give it an 'A' for creativity and 'A' for industrious."

He spotted some red flags along the course, however, saying that the cable that pulls the cart up the peak is “very small to put that amount of weight up the hill. Sometimes it just snaps and you can run the risk of it breaking, and smack a person in the face."

Read: Child Airlifted to Hospital After He Tumbles From Rollercoaster at Amusement Park

Another concern was the cart assembly, the steel of which "may be questionable."

"Would I put my 3-year-old on it? Probably not," Reiss admitted.

Watch: The Longest Yard: Meet the Families Building Makeshift Theme Parks at Their Homes