Exercise 1: Default Pod Networking, and What Changes Once a NetworkPolicy Selects a Pod — Possible Solution
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Default behavior, before any NetworkPolicy exists:

Per the chapter, "by default, every pod in a Kubernetes cluster can
communicate with every other pod, across any namespace, with no
restrictions at all." This is a FLAT, fully open network -- any pod
can initiate a connection to any other pod, regardless of namespace,
labels, or application boundary, with nothing blocking it at the
network level.

What changes the moment a NetworkPolicy selects a given pod:

Per the chapter's own tip-box, "the instant any NetworkPolicy selects
a given pod, that pod's traffic switches to deny-by-default, except
for what's explicitly allowed." This is a genuinely important and
easy-to-miss behavioral detail: the change in behavior isn't scoped to
just the specific traffic the policy MENTIONS -- it's a wholesale
shift for that pod, from "everything allowed" to "everything denied
EXCEPT what this (and any other) policy selecting this pod explicitly
permits."

This means creating a NetworkPolicy that ONLY specifies one allowed
source doesn't just ADD that one allowed source on top of the existing
open access -- it REPLACES the previously wide-open access with a
much narrower one, where ONLY the explicitly listed traffic is now
permitted, and everything else that used to work (even traffic that
has nothing to do with what the policy was trying to restrict) is now
silently blocked unless it was also explicitly allowed.

WHY THIS WORKS AS AN ANSWER
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This states both halves of the chapter's own explicit claim --
default-open before any policy, deny-by-default-except-explicitly-
allowed the instant ANY policy selects the pod -- and specifically
highlights that this is an ALL-OR-NOTHING shift for the selected pod,
not an incremental addition, which is exactly the nuance the chapter's
tip-box calls out as easy to get wrong the first time.