The empty fish tank.
If it's empty of water yet has mosquitoes, how do they fly?
Presumably it must be filled with both air and mosquitoes in carefully calculated proportions, too much air there would be wasted space in which you could have kept mosquitoes, too little air the mosquitoes will be crushed by their own buzzing mass..
How did the scientists resolve such a challenging conundrum.
Hmm... Air fish tanks delivered empty or full?