a biologist, an engineer and a mathematician were having coffee on the patio across the street as they watch two people walk into a house. a few minutes later, three people leave the house. they start to discuss how that could happen:

biologist: the two people copulated, reproduced and three people leave the house engineer: that’s wrong, our initial observation must have been erroneous mathematician: you’re both wrong. if another person enters the house it will be empty again

HA!