You get this for beating the 5th level. One of the more annoying levels, imo.
* Follow these steps exactly, because one of the coins for this level requires you do the part of the level on your first try *
1) There's a bump looking thing on the left, click it and it will move. Click it until it moves to the next area.
2) Pick up the rock on the ground in front of you. Go to the right.
3) Click the bump until it moves to the next area again.
4) Place the rock on one of the 3 circles on the ground. Go to the right.
5) Click the bump until it is above the zipper. Click the zipper. A rock will fall out.
6) Pick up the rock and place it on the circle on the right.
7) A statue will open on the left side. Take the rock from it. Go to the left.
8) Place the rock on a second circle on the floor. Go back to the right.
9) Pick up the rock from the switch on the right, and bring it back to the middle screen. Place it on the final switch.
10) You've opened a puzzle that will transform you into animals. There's a bee, a bear, and a mouse. Line up all 3 pieces of the animal you want, step in the glowing circle, and click the little fuzzy guy on top of the animal to transform.
11) To get a coin, make every incorrect combo of animals. All 24 of them.
12) Once you have the coin, transform into the mouse. Go to the left screen.
13) Click on the tree trunk to make it spin until the branch is pointing to the back. A little door at the base of the trunk should be open now.
14) Click the door to have the mouse climb up and pop out a cork. Go back to the animal changer, and turn back into the caveman.
15) Pick up the cork from in front of the animal changer. Go to the right screen.
16) Place the cork in the green thing's butt. Go back to the middle screen.
17) Now change into the bee. Go to the left screen.
18) Click the blue bubbles on the bottom of the screen. This will make you're bee have pollen on it.
19) Click the orange flower in the back to pollinate it, causing it to bloom. Go to the rightmost screen.
20) Do the same for this orange flower. The frog will try to eat you, but will explode because you corked him earlier.
21) Click the orange flower one more time to pollinate this one. Back to the animal changer.
22) Change into the bear now. Go to the left screen.
23) In the lower left corner, there is a rock that you can click to pick up. Bring the rock back to the middle screen.
24) Trade the rock for one of the small rocks you placed on the switches earlier. Bring the small rock to the right screen.
25) Place the small rock on the switch to open the statue again. Go back to the middle screen.
26) Pick up the big rock again. Go to the right screen.
27) Place the big rock in the statue's hand. Retrieve the small rock and go back to the middle screen.
28) Replace the little rock, and head to the left screen.
29) In the middle of the area, there's a rock with a paw print on it. Click it to stand on the rock.
30) You can now turn the tree and have it smack the bear. Smack the bear 3 times. To make sure that you have it right, you can go back to the middle screen, and the other tree should be pointing to the right while the left tree is pointing to the left.
31) Before moving one, make sure the path is set up so you can get the last coin. You should have
Left Screen: A bloomed flower, the tree pointing to the left
Middle Screen: Tree pointing to the right
Right Screen: Statue holding up a big rock, a bloomed flower
32) If everything is correct, change back to the mouse. Go to the left screen
33) Click the hole in the lower left.
34) This is a steady hand puzzle. You cannot touch the sides or else your cursor will reset to the beginning. Navigate to the end to click the arrows to leave the hole. Make sure to pick up the second coin in the top right before leaving.
35) If you followed this guide correctly, the mouse will go all the way to the firefly and you'll get the last coin for doing it on your first try!
36) Change back to the caveman and click the firefly to finish the most annoying level so far!