If the museum visitor stands in front of the exhibit less than 'td' seconds then presume that the visitor does not like the exhibit.
If the museum visitor stands in front of the exhibit more than 'td+1' seconds and less than 'tl' seconds then presume that the visitor likes the exhibit with an amount of liking following an fl(t) progress function.
If the museum visitor stands in front of the exhibit more than 'tl' seconds then presume that the visitor likes the exhibit with an amount of liking equal to 1.