1)What's this? It's a ball
2)What's this? It's a kettle
3)What's this? It's a train
4)What's this? It's a chest (cундук, не видно.)
5)What's this? It's an airplaine
6)What's this? совсем не видно.
Но если слово начинается на гласную то, an. Если на согласную то а.