Assimilation is a process where sounds in separate words change when they are put together in speech. One way this happens is by the second sound changing to be more similar to the first.

Example
The phrase 'white bag', which becomes 'wipe bag' when it is said.

In the classroom
Raising awareness of assimilation is often easier to achieve than actively encouraging learners to produce it, as it is natural process as fluency increases. One way to help learners notice is to use phonemic script to describe the changes to sounds.