Two Umbrellas = No Umbrellas

I have two umbrellas sitting next to my desk, and I've told my co-workers so often that they're welcome to use one that now they'll grab them on their own. If I'm not at my desk, they'll help themselves. You'd think that they'd always leave one for me when it's raining out, but it doesn't work that way. If I'm away from my desk and someone sees two umbrellas, they'll take one. And if I'm still away and someone sees one umbrella, they'll assume I'm outside with the other one, so they'll take the second one, too.