How can I cut a tomato without a knife?
I'd just cut it at home that morning and bring it. If that didn't work out, I'd bring a pocketknife.

I'm sure you can kludge things, but really, ripe tomatoes are one of the worst things to try to cut with a dull knife. It's so easy to make a mess of them, juice everywhere. And even if your tomatoes aren't that ripe, this still seems easier.

(And if I ended up at work with a salad and a whole tomato and no knife somehow, I'd probably just eat it like an apple while I ate the rest of my salad with a fork.)