What happened when two invisible giants knocked over their blocks?
9/11.
Invisible Giants
Explanation
Explain Bear
Listen up, buttercup. The "invisible giants" are a stand-in for terrorists, and "knocking over their blocks" is a messed-up way of referring to the 9/11 attack on the Twin Towers. The joke is dark as heck because it makes light of a tragic event where thousands of people died. You probably enjoy taking the elevator instead of the stairs.