Show a small white box with a dry erase board inside. Your spectator writes down the esp symbol they have in mind and closes the lid. The lid is bound by rubber bands and shown from all around, there is no way to see what was written. You are now able to read their mind. Looks impossible.
The secret is very ingenious
Very easy to make and to present
No electronics
Can be presented as many times as you want
Fully automatic
Very impressive and magical
A very complete fundamental piece for all mentalists