What number should come next in the sequence 6, 11, 21, 36, 56, ______?

Let's find the differences between consecutive terms in the given sequence:
* $11-6=5$
* $21-11=10$
* $36-21=15$
* $56-36=20$

The differences form an arithmetic progression: 5, 10, 15, 20.
Following this clear pattern, the next difference must be:

$$20+5=25$$

Therefore, the next number in the sequence is:

$$56+25=81$$