ampersand, followed by "lt", followed by semicolon (a "less than" symbol)

Then a b.

Then another ampersand, followed by "gt", finally another semicolon (a "greater than" symbol)

That gets you the first "bold" tag.

Then, type "bold"

ampersand, followed by "lt", followed by semicolon

Then a "/b".

Then another ampersand, followed by "gt", finally another semicolon.

That gets you the closing "bold" tag.

Following that sequence will get you something like uninterpreted HTML.