Cellos are the ones where the player is sitting down and the instrument is between their knees.
Sir Thomas Beecham (English conductor) once reprimanded a lady cellist thus:
"Madam, you have between your legs an instrument capable of giving pleasure to thousands and all you can do is scratch it."
Apart from the length and weight, one reason you can't play a 'cello under your chin like a violin is because it has a ruddy great spike in the bottom!