How do they get that foamy top on the beer?

Update: to be honest I am not much of a beer drinker, but I do like the foamy top, but most times there is hardly any foam or it collapses very quickly.
How can I get more foam, what do I ask for?
What is the maximum foam is it 100% or can it be higher?
If they had an all foam beer I would drink it more often
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