中 middle

Let's call the vertical line your tongue. So you could say,
Your tongue is sticking out the middle of your mouth, and you are CHEWing on it. (FYI you are epileptic)

the world ★★★★☆
世 (society) + 中 (middle) = 世の中 (the world)

the social, everyday world - more poetic than 世界。 Used in dramatic declarations like, "I'm sick of this fuckin' world!" "I don't care about this life!" "Poor Kurt Cobain, he was just too sensitive for this cruel world!" Also, usually implies JUST Japanese society.

the very center ★★★☆☆ KUNKUN
真 (really) + 中 (middle) = 真ん中 (the very center)

middle school ★★★☆☆
中 (middle) + 学校 (school) = 中学校 (middle school)

central ★★★☆☆
中 (middle) + 央 (central) = 中央 (central)

central. Tokyo has a famous train line: 中央線、(the central line).

intermediate-level ★★★☆☆
中 (middle) + 級 (level) = 中級 (intermediate-level)

stop/cancel ★★☆☆☆ NP
中 (middle) + 止 (stop) = 中止 (stop/cancel)

(temporarily) stopped or (permanently) canceled. This is a word you'd hear in an announcement. You wouldn't say to your friend, "I'm going to 中止 watching TV for a week."

in the middle of xxx ★★☆☆☆ SUF

in the middle of doing XXX.

XXX is always a noun: 電話中　（電話中）means 'I'm on the phone right now!'