Learners struggle in thinking of the words that they haven't learned yet. There are just as many words in Japanese as there are in English: this causes a serious problem no doubt.  

• イオン　Ion
  
• オランウータン　Orangutan
  
• 水素　Hydrogen
  
• 砂利　Gravel
  
• 暴徒　Mob
  
• 亜鉛　Zinc
  
• モップ　A mop
  
• 台形　（American) trapezoid
  
• ゾウムシ　Weevil
  
• 雀斑　Freckles
  
• 切り株　Tree stump
  
• 垂涎　Covetousness
  
• スレート色　Slate color
  
• 靴墨　Shoe polish
  
• メラニン　Melanin
  
• 彗星　Comet
• 放蕩 Debauchery
• 加速　Acceleration
  
• 造幣局　A mint (money)
  
• バイソン　Bison

These are 20 random words to fill 20 random holes that you didn't know you needed to fill in your Japanese vocabulary!