Day 006 The Shortcut Tax
Line counting, state tracking, and the cost of reaching for someone else's answer.
K&R
Day 005 The Program Listens
C stops printing and starts listening. getchar, putchar, and the precedence trap that has ruined code for fifty years.
Day 004 What They Didn't Know
K&R section 1.4. Symbolic constants. The preprocessor. And the realization that every layer was built by people who could not see what was coming.