Pues me he puesto a buscar y he visto varios sitios más o menos exóticos, pero al final la página que más me ha gustado es la wikipedia Mathematical jokes
An often-repeated joke is that topologists can't tell the coffee cup from which they are drinking from the doughnut they are eating, since a sufficiently pliable doughnut could be reshaped (by a homeomorphism) to the form of a coffee cup by creating a dimple and progressively enlarging it, while shrinking the hole into a handle.