- "The Thinking Cap... I'm gonna help you finish the Thinking Cap."
- "It's an Intelligence Booster, actually. It's got an intelligence boost."
- —Cisco Ramon and Harry Wells[src]
The Thinking Cap is a device conceptualized by Clifford DeVoe and built by Marlize DeVoe to help make the former smarter. A modified version, nicknamed "Death Cap" by Cisco Ramon or "Intelligence Booster" by Harry Wells, was developed by the two of them in order to bring Harry's mind to the same level as DeVoe's in an attempt to get ahead of him.
Wishing to enlighten his students' minds, but unable to execute his plan, Clifford DeVoe designed the concept for a piece of technology he described as his "thinking cap", a device that would allow a human to use the full capacity of their brain. He requested that his wife, Marlize DeVoe, create it for him, citing her competence of mechanical engineering.
On the night of January 7, 2014, the couple attended the public unveiling of the S.T.A.R. Labs particle accelerator. Anticipating the failure of the construction and the subsequent explosion, and hoping to siphon the excess energy to power their device, the duo remained on the premises following the conference. As the accelerator exploded, DeVoe stood in the rain, allowing the cap to absorb numerous bolts of lightning generated by the accelerator. Coupled with the release of dark matter from the particle accelerator's explosion, DeVoe was transformed into a meta-human wielding a highly enhanced intelligence, due to the introduction of the Thinking Cap.
A second version was built at the insistence of Harry Wells and reluctance of Cisco Ramon as a way to better combat DeVoe. Cisco was more worried over the long term effects it would have on the user, aware that the original cap tainted DeVoe. At Cisco's order, Harry altered his design to operate to reduced capacity without the introduction of Dark Matter. Despite multiple setbacks, including setting itself on fire, Harry got his Thinking Cap to work, using it to teach Iris to make a tidal wave while empowered with Barry's abilities, and to learn the names of the last two bus-metas: Janet Petty and Edwin Gauss. 
Behind the scenes
- In the DC comics, the Thinking Cap was the main source of the Thinker's abilities, which included telekinesis and mind control.