Take care with a knife
Materials you need are:
a large container with a lid
large plastic bag
Put grass clippings into the large container and cover it with an airtight plastic bag to collect the gas that is made. Leave the container in a warm place for a week and observe the fullness of the plastic bag.
As the plant matter is decayed by the decomposing bacteria, there is an increase in methane gas which is a flammable hydrocarbon.