Abstract: It's well-known that there is only one line passing through two distinct points in C^3. In the pre-colloquium we will apply this fact to solve the following more complicated conditions, with the aid of Chern classes:
  1. How many lines passing through a fixed point and intersecting two fixed lines in C^3?
  2. How many lines intersecting four fixed lines in C^3?
  3. How many lines lying on a cubic surface in P^3?