How does instantaneous speed differ from average speed?

Average speed is the mean speed of an object all over the journey, it is calculated by dividing the total distance covered by total time taken .

where as,

Instantaneous speed is the speed of an object at a particular instant of time. it is calculated at an instant by using minute values using differentiation (calculus).

i.e

obesity is a result of eating unhealthy junk food which increases the fat content of the body and makes you obess. it is also further because of not doing proper exercise and not participating in any physical activity.

what do we need our phalanges for?