Both penguin datasets are already tidy! 
We can pretend that penguins wasn’t tidy and that it looked instead like untidy_penguins below, where body_mass_g  was recorded separately for male , female , and NA  sex  penguins.
<-  penguins |>  pivot_wider (names_from =  sex, values_from =  body_mass_g)
# A tibble: 344 × 9
   species island    bill_length_mm bill_depth_mm flipper_length_mm  year  male
   <fct>   <fct>              <dbl>         <dbl>             <int> <int> <int>
 1 Adelie  Torgersen           39.1          18.7               181  2007  3750
 2 Adelie  Torgersen           39.5          17.4               186  2007    NA
 3 Adelie  Torgersen           40.3          18                 195  2007    NA
 4 Adelie  Torgersen           NA            NA                  NA  2007    NA
 5 Adelie  Torgersen           36.7          19.3               193  2007    NA
 6 Adelie  Torgersen           39.3          20.6               190  2007  3650
 7 Adelie  Torgersen           38.9          17.8               181  2007    NA
 8 Adelie  Torgersen           39.2          19.6               195  2007  4675
 9 Adelie  Torgersen           34.1          18.1               193  2007    NA
10 Adelie  Torgersen           42            20.2               190  2007    NA
# ℹ 334 more rows
# ℹ 2 more variables: female <int>, `NA` <int> 
 
 
Now let’s make it tidy again! 
We’ll use the help of pivot_longer()
|> pivot_longer (cols =  male: ` NA ` ,           names_to =  "sex" ,           values_to =  "body_mass_g" )
# A tibble: 1,032 × 8
   species island    bill_length_mm bill_depth_mm flipper_length_mm  year sex   
   <fct>   <fct>              <dbl>         <dbl>             <int> <int> <chr> 
 1 Adelie  Torgersen           39.1          18.7               181  2007 male  
 2 Adelie  Torgersen           39.1          18.7               181  2007 female
 3 Adelie  Torgersen           39.1          18.7               181  2007 NA    
 4 Adelie  Torgersen           39.5          17.4               186  2007 male  
 5 Adelie  Torgersen           39.5          17.4               186  2007 female
 6 Adelie  Torgersen           39.5          17.4               186  2007 NA    
 7 Adelie  Torgersen           40.3          18                 195  2007 male  
 8 Adelie  Torgersen           40.3          18                 195  2007 female
 9 Adelie  Torgersen           40.3          18                 195  2007 NA    
10 Adelie  Torgersen           NA            NA                  NA  2007 male  
# ℹ 1,022 more rows
# ℹ 1 more variable: body_mass_g <int>