[Casiopea]

It's called palatalization.

The letter "t" is pronounced as an affricate, [ch], before a high vowel:

future
statue
congratulations

(Note, /t/ is often voiced to [dj], as in jump: congra[dj]ulations

Also, the letter "t" is pronouced [ch] before "i" and "e":

righteous (Note, phonetically "e" is [i])
Christian
question

Notice also the sounds that come before "t". In our three examples above, fricatives "gh" and "s" come before "t". Look at words where "t" is preceded by a stop, or plosive,

caption
attention

/t/ is pronoucned [sh].

Look for words that have medial "ti" and "te", preceded by fricatives. That's a start.