Some people don't know what to do when a new person comes in. Ignoring them is not welcoming and they will probably leave with a bad impression. While the lab is open you should try to do your best to be friendly. If you don't know what to do here is a suggested procedure.

1. Look up from your computer and say hello.
2. Ask if they have been before, what their interests are, why they have come.
3. Offer them tea or another drink.
4. Offer them a tour.

If you can't face this and there is noöne else in the lab who will do it then it might be worth ending the open night.