Number One : Playing with words or small phrases in my head.
I've had this habit for nearly as long as I can remember. I don't know exactly when or why it started but I have a vague memory of the period in my life. Basically, I might take a short sentence (it could be anything) and count the number of leters in it. Even numbers or numbers besides prime numbers are good. Then I'll chop it up into smaller and smaller bits, repeat parts backwards and try saying the word (or part of word) as though it can still be said and understood but with fewer letters (usually omitting vowels first and leaving the strongest consonant til the end). I'm so 'skilled' at it that the process is pretty quick for me but to include it all here will take ages so the following example is only a fraction of the entire event.
Example Word (but could be a sentence): 'I saw the elephant' (15 letters)
Thus: I saw t (5) - he ele (5) - phant (5)
Then: I saw t
»is, si, s, s
» isa, asi, sa, as, s, s
» wt, tw, t, t,
»awt, twa, wt, tw, t, t,
And so on...
It will even go a step further than this to the point that there's no letters left, just a mere grunt in my head (a sound that is supposed to mimic the absence of sound), and this is usually repeated backwards as well. ie. er, er. Am I CRAZY or WHAT!?
crazy, yzarc, crzy,yzrc, crz, zrc, cz, zc, z, z...