Yesterday morning ,i got a phone call from my best friend . She said that today is my birthday , i 'd like to host a party .will you come ?" okay , i am willing to celebrate it for you .after i finish my work , i will come over soon. " I lock off at 6:00 pm ,then i went to my friend's home by taxi . When the car on the way , it didn’t work because of low battery . I am worried that i would miss the party so i decided to ask other taxi .But the driver said :"please don't worried , i have an emergency tool-car jump starter ,it can solve this problem " I am surprised , cause of i have never hear that tool before . To my satisfaction , the car was finally moved after charging ,i arrived at the party on time . After i got off , i still wonder what is car jump starter ,does it like the power bank for mobile phone ?