Question

What happens when we light the incence stick in one corner of a room

Answer 1

When we light an incense stick it's particles starts moving and the kinetic energy increases.
The particles intermix with the particles of air.
The concentration of these particles is high in that corner and less in the other parts of the room
As particles move from a region of high concentration to a region of low concentration, the particles spread in the room till equal concentration is achieved.
therefore when incense stick is lighted we can get it's smell in all parts of the room.

Answer 2

When we light an incense stick at the corner of a room, it releases gaseous particle which due to their high kinetic energy, spread in the whole room and hence we are able to have its fragrance through out the room.
Hope it helps