They do not know why da Vinci painted it, or how it ended up in the collection of French ruler Francis the First.

VOA: special.2010.06.23

Mister Weindruch says researchers do not know why reducing calories increases the length of life.

VOA: special.2009.09.01

They do not know why all these things were found together in one place.

VOA: special.2010.08.03

Scientists do not know why women are more at risk than men.

VOA: special.2009.04.14

Researchers do not know why happiness increases as people get older.

VOA: special.2010.08.03

I don't know why I do it. I just do it."

VOA: special.2009.07.04

Scientists do not know why women are more at risk.

VOA: special.2010.06.15

I do not know why I should fear death.

VOA: special.2010.07.10

Doctors do not yet know why this is true.

VOA: special.2010.06.15

What, you think I regret saving you? I can see that you do.I just... I don't know why.

VOA: standard.other

"Do you know why they targeted you?"

VOA: standard.2010.05.18