The knee — the joint in the human (or animal) leg used for bending while standing or walking.
子供が転んで膝をすりむいてしまった。
The child fell and grazed their knee.
膝が痛くて長時間歩けない。
My knee hurts and I can't walk for a long time.
高齢になると膝の関節が弱くなることが多い。
As people age, knee joints often become weak.
The lap — the area formed by the thighs when sitting; used for placing things on or being held on someone's lap.
赤ちゃんを膝に乗せて絵本を読んだ。
I put the baby on my lap and read a picture book.
コートを膝にかけて寒さをしのいだ。
I covered my lap with a coat to keep warm.
犬がいつも私の膝の上で眠る。
The dog always sleeps on my lap.