When transferring a colony from an agar plate to a slide why

When transferring a colony from an agar plate to a slide, why do we not place the lid of the plate on the counter?

Solution

While transferring a colony from an agar plate to a slide, we should not place the lid of the plate on the counter to prevent contamination. While transferring the colony we will place the lid on the top, against the counter, so that we minimize the chances of contamination.

The petri dish lids are made in such a way that it can prevent the falling of dust on the plates, but it will allow the diffusion of air around the edges, like this, plate will never have direct air current, always we need to keep the plate covered with lid, it acts as a shield, so never keep the lid on the bench top, otherwise all the dust particles will try to enter inside the Petri plate resulting in contamination.