Example 2: When A was B's current age, B was 10 years old; when B was A's current age, A was 25 years old. Who is older, A or B? How many years older?
Solution: Let the age difference between A and B be $k$ years, which is an undetermined constant. When $k>0$, A is older than B; when $k<0$, A is younger than B. Then, A's age $y$ and B's age $x$ have a linear relationship:
$$
y=x+k \text {. }
$$

After designing this dynamic process, the given conditions become 3 "moments" of this process:
(1) The current ages of B and A are $(x, y)$;
(2) "When A was B's current age, B was 10 years old", which is equivalent to taking $(10, x)$, i.e.,
$$
x=10+k \text {; }
$$
(3) "When B was A's current age, A was 25 years old", which is equivalent to taking $(y, 25)$, i.e.,
$$
25=y+k .
$$

By combining (1)+(2)+(3) and eliminating $x, y$, we get
$$
k=5 \text {. }
$$

Answer: A is 5 years older than B.
2. Solve local problems in a global context