Four ships are sailing on a 2D planet. Each ships traverses a straight line at constant speed. No two ships are traveling parallel to each other. Their journeys started at some time in the distant past. Sometimes, a pair of ships collides. A ship continues its journey even after a collision. However, it is strong enough only to survive two collisions; it dies when it collides a third time. The situation is grim. Five of six possible collisions have already taken place (no collision involved more than 2 ships) and two ships are out of commission. What fate awaits the remaining two?
This puzzle is obtained from https://gurmeet.net/puzzles/four-ships/index.html
I found a similar question online Ghost Ship Collisions but this visualises a tetrahedron and thus says that the ships won't collide further but I cannot think of any 3D figure smaller than tetrahedron that has 4 edges, thus I am stuck.