をためる

to save up; to accumulate

→ を
phrase Uncommon #19,132
to save up; to accumulate The particle を marks what is being saved or accumulated.
JA: 万一の場合に備えてお金をためるのは賢明だ。
EN: It is wise to save up money in case of an emergency.

家賃をためるのが楽しいのかい

Do you enjoy racking up unpaid rent?

— カンフーハッスル (2004)

ラクダは背中のこぶに大量の脂肪をためることができる。

Camels can store a large amount of fat in the humps on their backs.

ラクダは背中のこぶに大量の水をためることができる。

Camels can store a large amount of water in the humps on their backs.