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My colleague leaves the bathroom without washing his hands. How can I let him know without offending him?

Try a non-confrontational approach first, like the strategy used by University of Denver researchers. They found that people were more likely to respond to signs in the bathroom that mentioned how gross it is to have "faeces" and "urine" on your hands. "People don't respond to overused messages about spreading germs but they do pay attention to explicit wording like this," says researcher Renee Botta, PhD. "Plus, the threat in the signs starts others talking and people also respond well to social pressure."

You can also try coerce your co-worker into rinsing his mitts by giving him a friendly dig: "Hey, wash up man! Do you know where your hands have been lately?"