Test case for Ball pen:
1.successfully check the ink in the pen
2.successfully check cap whether it is comfortable r not
3.successfully check the Pin point(Ball point)
4.successfully check the colour of the pen


Test cases for Water Bottle:
1.successfully check water is in the bottle r not?
2.successfully check the cap of the bottle
3.successfully check the bottom 
4.successfully check compatibility

Test cases for pen
1. height, width, volume of pen
2. the max no of characters that pen can write
3. test the behaviour of pen in extreme conditions like 
heat, cold ,rain
4. test the apt ange of the pen with paper for writing
confort level while holding the pen
.......
Water bottle:
1. volume of bottle
2. volume of contents of bottle
3. other properties of bottle

To write proper test case one must need requirement 
document then we can tally with that whether the test cases 
covered the required or not

To write proper test case one must need requirement 
document then we can tally with that whether the test cases 
covered the required or not

To write proper test case one must need requirement 
document then we can tally with that whether the test cases 
covered the required or not

Test case for Ball pen
1)check integration between ren nipe point and ink flow
2)check integration between cap and pen body
3)check ink quality
4)check refile quality and its integration with pen body

Test case for water bottle
1)check capasity of bottle
2)check integration between cap and bottle
3)check shap of the bottle
4)check quality of material to be used

I think Only Sriram write the correct ans .

testcase for water bottle: 
  verify it is one litre bottle or two litre bottle
   verify bottle color is good or not
   verify any hole or broken is thr in bottle
   verify the cap is thr or not
   verify the shape of the bottle 
   verify the quality of material to be used or not
test case for pen:
 1) verify the which type of pen (its ball pen or inkpen)
 2) check pen is writing or not
 3) check  color (it is blue or black or etc)
 4) at the wtiting itis writing smoothly or not
 5) check height,width,

u write test cases of the any object like pen,water 
bottle...etc you need to specifications of objects.

with out specifications we can not write the testcases.

for ex:u take Pen Object

Test cases are:

1)Look and Feel :width,height,color,design etc.

2)performence:we need to check  that object It is writting 
or not (performence)

3)usability:what is the use of this object

4)Rigidity:strong ness of the object.

these things we can follow to write test cases of the object

For Ball Pen:

Use ball pen in flight. i.e. test with various environment 
condition i.e. low pressure,tepmerature etc.