Explanation: Ah, yes! A bad bilingual joke. Bullfighters are often found in Spanish speaking countries. The number four in Spanish is “cuatro.” If you are in quicksand you are probably sinking down into the sand.
Many English speakers who do not speak Spanish play with Spanish by adding an “o” onto any English word- so “sink” would become “sinko.” Of course “sinko” sounds just like “cinco” which means five in Spanish. This joke is funny to an English speaker who knows a bit of Spanish because the answer sounds like 4, 5 in Spanish as it refers to the 4 bullfighters sinking into quicksand.