Irony is the use of words that are the opposite of their intended meanings. The example provided above is an example of;
The example provided above is known as situational irony. Situational irony occurs when the results obtained during an event are the opposite of what should be expected.
In the case of the English settlers described, situational irony is the case because the passage reported that they took good care of their teeth, but the findings from the archaeologists show that their use of tooth powders resulted in tooth decay.
The opposite of the intended result is therefore the case. So, this is situational irony.
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