The idea to make a device which eventually ended up exploding on a Tube train last September was hatched during a summer of boredom and became a cry for attention, a court has heard.
The object was never meant to explode or cause harm, but instead "just burn", its maker Ahmed Hassan said.
He told a jury he had tested it and taken steps to ensure it did not have deadly consequences.
Before bringing the 400g bomb onto a District Line tube he tested 50g of TATP on a Coca Cola can on the kitchen table of his foster home in Surrey, he said.
He told jurors: "I heard a whoof and a very small heat, fire, for a fraction of a second.
"I was certain that it would not explode. It would just burn."
He added shrapnel because he "wanted it to look serious".
Hassan told how he stayed awake all night on a sofa in the conservatory the night before the explosion, and had the device at his side when his foster father Ron Jones approached him to ask him why he was not sleeping.
He became attracted last summer by the idea of becoming a fugitive, having spent hours watching action movies and documentaries, he said.
On why he made the device he said: "I think the main reason, I wasn't thinking as a normal person would do. I was very bored, very stressed, very confused and I watched lots of movies, action movies during that time."
He added: "It became kind of a fantasy in my head. I was thinking about it. Yes, that was it.
"I was watching documentaries as well, about fugitives and just the idea of being a fugitive got into my head. And I thought about it and that was it."
Tim Moloney QC, representing Hassan, asked: "Was that an attractive idea?"
The defendant answered: "Very much so."
The 18-year-old described the summer holidays as "basically my enemy".
He told the Old Bailey how he had been unable to sleep due to bad dreams, had had suicidal thoughts and admitted hurting himself in the past.
He thought about making the device about four weeks before the incident, he said.
An online search for explosives led him to come across TATP which was described as "homemade" and "very easy".
He looked up the chemicals to make it and watched YouTube videos demonstrating small amounts of it, some in front of small children, he said.
Under cross-examination Hassan denied wanting to hurt or kill anyone, saying his fantasy was about being a fugitive and that he wanted "attention".
Insisting he never tried to blame anyone else, he said: "I left evidence all over the place ... I left everything behind to suggest it was me. I actually was seeking them to find out it was me."
The idea of killing someone had "never crossed my mind at all, never in my life" he added.